Since I'm over 100K miles this weekend was maintenance time. Figured I would change all my fluids (gear boxes) as part of the usual service, and share my observations. The transmission and T-case intervals were not due yet but I figured if I have the time now...
Engine: switched to Mobil1 High Mileage oil, keeping a 10K mile interval. Oil was black but no metal evident in the old oil. I change oil filters every 5K. I prefer Fram double guards.
Front Dif: first change. Oil was dark but not black. 9mm hex bolts were tight. Drain plug was very clean. Used Mobil1 75-90 to refill.
Transmission: first change on the 6 speed manual. Used Lucas full synthetic 75-90. Old oil was dark with no metal flakes but the magnetic drain plug had a some dark material stuck to it, looked like dirt. Not much of it though, maybe a half teaspoon.
T-Case: first change. I thought maybe I had gotten drunk one night and changed the T-case oil earlier since it was so clean (it could happen). Seriously, looked new! Have a piece of cardboard handy to keep the drained oil off the exhaust crossover. I used 75-90 Lucas full synthetic here too. Again, small amount of "dirt" on the drain plug magnet but no metal flakes or shine.
Rear dif: 2nd change, first was at 60K (I think). Oil was very black on draining. No metal showing in the oil, just a dirty drain plug. Used Mobil1 75-90 here as well.
Observations: no need to change t-case and transmission early, but no harm either. Removing and re-installing Budbuilt skids is a pain, but I did find that 12mm socket I have been missing for a year, so I've got that going for me.