Sounds like NORMAL behavior for an aftermarket thermostat. What kind is in it?
From what I've seen, only the OE Motorcraft 'stats and some of the thin, wimpy ones regulate the temp in such a manner that the temp gauge needle goes to the middle and stays there. The rest (Stant, Stant Superstat, etc.) don't open until the needle is past the center, then they open and the coolant rushing through causes the needle to go to the left of center, and then once coolant is flowing normally it just stays in the middle or goes a little right or left.
It also has a lot to do with the operating temp of the 'stat in question. Most aftermarket thermostats for the 4.0L are 195 degree, which is 5 degrees higher than the OE Motorcraft, which opens at 190 degrees. That might not sound like much, but that extra 5 degrees until it opens is what makes that gauge needle go that little bit farther over to the right before it opens and goes over to the left again. There are also 'stats that are as high as 198 degrees.
I'd say going to the AL in NORMAL is kind of high, but if it does that and you haven't had any problems, maybe it's just a 198 degree thermostat, or a defective unit that isn't opening correctly but still gets the job done. I've had enough fun with the cooling system that I just change the 'stat out when the needle starts going over a little too far, because that usually means it's on it's way out and will eventually get stuck closed. The consequences of a stuck 'stat range from mild hassle having to change it in the field, to catastrophic overheating of the engine and being stranded. Neither is worth the risk compared to the ~$5-20 it costs for a new one and the time it takes to swap it out or drain and refill the system.
When it comes to the cooling system, your best bet is to use distilled water (99 cents a gallon at the grocery store) and a 50/50 mix with coolant. I would also suggest using something like ZEREX Green (good 'ol Ethylene Glycol) or ZEREX G-05 (Same gold-color stuff they use in all new Fords) coolant. I used Prestone for the longest time, but their most recent "All Vehicles" formula is basically DEX-COOL. I had a water pump gasket leak and hose clamp leak that nothing would solve. I switched to ZEREX G-05, and haven't had a leak since. DEX-COOL type coolant seems to be harmful to the gaskets in these and other vehicles.
