if you get into the twisties with the quadradrive system, the rear wheels will have the first shot at getting traction. If one of the rears starts to spin while the other has traction, the varilock diff *should* start to lock up after a few wheel rotations of the spinning wheel. After that, if both rears are spinning, the t-case clutch should start to lock up and get the fronts spinning and the front diff should then act similar to the rear. It requires a bit of wheel spin to get both diffs locked up and working. Unless you have way oversized/heavy tires (or worn out clutch packs or too much friction modifier fluid in the varilock diffs), the system should work well.
If you have a non-varilock rear diff, I'd think the transfer case would still work since your rear driveshaft would be spinning faster than the front if the rear axle doesn't have traction on either tire, so really the only way to tell if you do have a varilock diff in the rear would be to open it up to look, or put the jeep up on jack stands and try to rotate one of the rear wheels by hand. If it begins to lock up after a couple rotations (you can definitely tell if the system is working correctly), you have a working varilock. If it doesn't lock up, you may have a non-varilock diff or a varilock diff that is worn out or with too much friction modifier and no longer works.
If it were me, I'd try putting it on jack stands and spinning the wheels by hand. If the rear doesn't lock up, I'd probably just open the rear diff to see what's in there and do a fluid change while I was at it. I suppose there may be a way to tell if you have the vari-lock diff by peeping through the diff cover fill hole too if you knew the visual differences between the setups.
