I've been driving this vehicle with the front driveshaft discconected for almost a year...My theroy is that I waited too long to get the transfer case issue fixed and driving with the front driveshaft connected too long that it totally screwed up my front differential. I think it was just too much abuse being in FULL AWD all the time for the front dif and it couldent take it anymore. thats why I THINK the seal blew out. It was just too much straign. It is still hard for me to belive that I've had these problems at such low miles (~35k when problems started) I don't think the front diff had issues FIRST causing the Transfer Case to be screwd up...It makes makes more sence the other way around.
From what I have read (and I have not read all of it) it sounds as though you had viscous coupling issues when you bought the truck. You bought it with differing diameter tires on it. This will cause the front and rear axles to turn at differing speeds. The viscous coupling will do its job and will try to correct that. It "monitors" rotational speed differences front/rear using friction/heat. If one output shaft in the transfercase is turning at a different speed from the other it will cause the VC (viscous coupling) to slip, causing heat. The heat will in turn lock the VC, which in turn will lock the front and rear output shafts together so that they WILL turn at the same speed, even if the tires have a different diameter. The VC will lock up tight enough to literally cook the oil in your transfercase, your front and your rear differentials.
Heres what happened to your truck:
1) Bought it with bad tires, had VC issues you were not aware of
2) Drove it the way it was (or previous owner did) for a period of time. This was long enough to literally weld your VC together
3) You replaced tires with same diameter ones, lengthening the time your differentials will last, but t/case is still junk.
4) Drove truck with junk t/case. Every time you turned the wheel you caused excessive heat to build up in the front differential.
5) Somewhere down the line something melted the plastic cap on the front diff vent tube. That act caused the entire front diff to be sealed, heat caused the diff oil to expand, excessive pressure found its way out via your axle seal.
6) You replace t/case.
While replacing your t/case may make you think that your problems are over, they are just beginning. Your whine noise that becomes apparent around 23 miles an hour means that one (or both) of your differentials need to be rebuilt or replaced. That noise is bearing related and (contrary to what some will no doubt post later) just changing your diff oil will not make it all better. Most likely the noise is coming from your pinion shaft bearings, which would make sense since they would take the brunt of the force being applied by a locked VC.
If you are a gambling man and are willing to take a little risk you can go to
www.car-part.com and shop for whichever differential is making the noise and just replace it. I would rebuild the one in the truck if it were me though.
How I learned this lesson was not as long and dragged out time-wise as you. I learned it over a span of just a few months. I replaced the front diff, both CV axles, rear axle assembly, all tie rods, upper and lower balljoints on both sides and both hubs, along with losing the AWD t/case and swapping in the BW4406 manual shifting t/case from a F-150. No more issues.
But keep in mind my truck is a second gen, I think the one you are having issues with is a third gen, but the same principals apply.
