That's a tall order. Don't know how much detail you want. I am also not familiar with the device you are measuring slipage with, so I'll try my best.
I always think of converter slippage in terms of RPM myself but we should start with some basics. The torque converter ties the engine output shaft to the transmission input shaft much like a clutch does. It provides the needed "slip" so the engine does not stall when the vehicle is sitting still. That is all it absolutely has to do.
Remember I am simplifing everything. If this is too basic sorry. If you want further installments with greater detail later, ask. The TC is a viscous coupling. Unlike a clutch where a friction plate makes physical contact with another plate and ideally the two rotate at exactly the same speed when fully engaged. With no slip at all. This viscous connection is transmission fluid. Think of it as two fans facing eachother inside an enclosure. One is turned by the enginge. The opposite one is connected to the trans input shaft. As the one blows air at the other, the air begins to make the opposite fan turn too. Just not at the same speed. This is called slippage. Now replace the air with trans fluid and put the two fans very near eachother. That's a basic torque converter.
Modern torque converters are all "lock-up" style. That is, the two fans can actually lock together under some conditions eliminating all slip. This is accomplished in a variety of ways. Oversimplify it, think of clutches engaging between the two fans so they are locked together. No slippage. This is much more efficient as there is little wasted energy. Better MPG, higher top speed and faster acceleration (back to the drag racers).
You must understand that any given TC, in any give application, under any specific load will have a different amount of slippage. If you add more power you get more slip. More load (more vehicle weight, better tire grip, lower numerical differential gear ratio) you get more slip. TC slip is one of the largest contributors to temperature rise in an automatic transmission. If you hold a vehicle still with the brake and apply max enginge power, the RPM that the enginge rises to is called the TC "stall speed". Add a more powerful engine and change nothing else and the "stall speed" for the TC will increase. Subtract enginge power and the "stall speed will decrese. All of those advertised stall speeds are just for comparison. Your results will vary.
At the beginning I said I would over simplify. Any TC will slip, even when locked if you apply enough power and enough resistance (load) to the trans input shaft. A clutch will also slip at some point. On unmodified street vehicles, this amount of slip is very small or now a days, non existant. Never fear. Look back in this overly long story and you will see I said the engineers found ways to gain additional advantages from this slip in the TC and only lockup the TC under specific conditions.
Back to drag racers. Add slip at launch and you get two advantages besides not stalling the engine. First, the engine RPM immediatly soars to a speed where the engine makes more power. Second, you get torque multiplication (remember I am over simplifying here) and thus more power to the rear tires. Drag racers often use torque converters with 3000 RPM stall speeds or more.
The TC can be locked manually (think springs and such that react to RPM) or by a computer. The manufacturers engineers decide how much slip and when based on the engine power production characteristics, the weight of the vehicle, gearing and the intended purpose of the vehicle to name a few. As you mentioned, in your particular application, the TC will not lock below 45 mph. Same truck but with a V8 (if they had the same trans) and this would be different. Typical stall speed of unmodified street vehicles may range from 1800-2400 rpm or so. My Explorer, making whatever power it curently makes has a stall speed of about 2600 rpm. Stock TC.
Acceterate moderatly up through all the gears maintaining a constant throttle application. Watch the tach and the rpms slowly rise and then abruptly fall with each transmission gear change. 1..2..3..4..5... Watch carefully and with older vehicles (less sofisticated TC's) and at the end you will see one more drop. That's the TC locking. More sofisticated TC's may engage as rpm rises in each gear only to disengage with an upshift, and then re-engage farther up the RPM scale. It all depends upon what the engineer designing the TC and then selecting the specfic TC for the specific application thought was best for the application. Some of my drag cars do not have computers controling the trans or TC yet they will lock once underway in first gear and never unlock again. Some of my higher power street vehicles that use automatic transmissions will do the same under wide open throttle but at less agressive throttle applications, may lock in each gear. Unlocking for the shift and then soon relock (they sense vacuum as a measure of load). Add a computer controller and you can do almost anything. Locking and unlocking, shifting up or down, whatever you think is best for the load detected.
The slippage percentage should be just as you said. The RPM difference between engine output shaft and transmisssion input shaft, expressed as a percentage. The only time it would ever be anywhere near 77% is under very large throttle application, regardless of transmission gear (1st, 2nd, 3rd, etc.). When it said the TC should lock at and above 45 mph, it means it will not lock below 45 mph no matter what. Under a light load the "two fans" will still spin near the same speed even without it locked. Under heavy load the speeds will be very different. Upto the TC "stall speed" regardless of road speed. Unless the engineer decided that it should lock or in some cases partially lock under certain conditions.
See how complicated this is. There is no easy answer. Once you leave the 70's or in some cases the 1980's most vehicles use electronic controllers to get ever increasing efficiencies out of the TC. They have enourmous tables of conditions, loads, temps, throttle position and almost anything else the vehicle is capable of sensing to determine what the TC should be doing. Locking, unlocking, partially locking. This is definatly not simplified nor can it be.
That one time you said you where going 55 mph and the TC was slipping 75%, what was going on. Where you still in top gear. Going up hill. Bucking a serious head wind? Anything that required a very large throttle application. I'd have to calculate it out from gear ratios but thats probably about the equivalent of downshifting 2 gears. At 55 mph was your engine really turning at about 3800 RPM? It could be. An engineer may chose to unlock the TC rather than downshift if the need may be temporary and short lived. This helps prevent that dreaded gear hunting sensation many vehicles experience.
As I've said, a very complex topic. Is there a real issue with your Explorer or where you just noticing things you really never thought about before? If something is still unclear, just ask and I will try to clarify. Next time in a much shorter answer.
