Yep, they are expensive. I had a stealership quote of $847 earlier this year. Aftermarket is $450-$600. Used ones can be found for less $50, but you have to trust how many miles are on it. You might even be able to get a unit from a junkyard for a decent price. It would be a toss up on pulling the VC or whole TC.
The operation of the 249 changed 95-96 (I'm not sure if that was the same as when the gear cut was changed) Some say the VC is different spec, others say it's the same. I don't know for sure.
Rebuilts can be iffy. Depending on the VC application, there needs to be the right spec fliud and a precise amount of air and in some they are pressurized. The air is so little in some cases that it gets tiny bubbles spread throughout the fluid, and may not be noticeable if you drain it.
How many miles? Depends on how many "lock" cycles and how many "slippages". The wear occurs during the slippage. Slippage occurs in normal driving during turns. Lock occurs when the slippage exceeds normal turning slippage. Lock is not true lock because there is still a little slippage then.
Hard to predict how many miles. I have heard ranges from 50k to 150k for VC's in general but it's gonna depend on how much it gets used. Other vehicles use VC's and the lower numbers are in designs which put the VC to use more often. They can be used in FT4x4/AWD and on-demand 4x4/AWD systems. They can also be used in a limited slip diff.
Two things are happening during the wear. The discs become slightly "polished" which means the small imperfections are worn off. Those tiny pieces of metal end up in the fluid. That gives the fluid more "grip". That extra grip is what causes the binding when a VC goes bad.
If the fluid was changed often enough, you would have completely polished discs and nothing for the fluid to grip. That's why a rebuilt can be iffy. A rebuilt (refilled) might last longer before binding, but it might not "lock" as hard. And that's assuming they get the right spec fliud and air and pressure(if any) for the application. Because of the slight polishing of the discs it will probably not be in manufacturer's specs.
Some designs have solid discs. Some have slots or holes in the discs. They don't contribute directly to grip, but do change how quickly the fluid begins to grip. BTW the fluid grips with the addition of heat caused during slippage. That's why a bad VC will not bind on short trips, not enough heat buildup. Longer trips have enough heat buildup to cause binding in the contaminated fluid.
Some tips I found for conserving the expensive VC.
-You must have the same size tire all around, even the spare. DC let some QT ZJ's out with a compact spare, there was a TSB.
-Keep the tires inflated so the actual diameter is the same all around. I used a tape measure. Depending on tire size and wear, you could find that you need a couple of PSI different pressures front and rear. Diameter is most important for VC longevity. Recheck if you go from empty cargo to heavy loaded cargo.
-If you get stuck, don't keep trying to get out if you get no or little movement. Every cycle of slip to lock causes that much more wear. Save it for when it will do ya some good. You do have tow points right?
The 249 has been labeled as bad for off-roading. It works for me. My open diffs are my problem not the VC. True lo locking is best and the later 249's have it. I suppose I would notice a problem with bigger tires and lower gears and some sort of diff traction aid.
