Check your oil pan. Chances are good that there is rust hole under the skid plate and its trying to impersonate a rear main leak. FTR, the shims on the T-case can be accessed through the cover that sits behind the rear output flange. Not hard to set up. I personally prefer RTV on the T-case union as opposed to the gasket since I hate doing things twice when a leak develops. There are O-rings on either end of the idler shaft that are must-dos. Beware, if you pull the T-case apart, try to leave the idler shaft in place or it will open up a passage for the tranny fluid to dump out in your crotch when you pull the rear of the T-case off. Take a good look at the idler gear's inner bore, the idler bearings and the shaft itself. That seems to be, at least for me, where things go south.
1985 FJ60, 350HP TBI 350, NV4500, NP241, 14-bolt semi floater rear w/Elocker, 4.56 gears, SOA, four-wheel disk brakes, 35" BFGs, 405,000 happy miles.