It sounds to me as if your center differential isn't transferring power - everything else is connected mechanically with gears or shafts so to gradually loose power to front and rear wheels it's pretty much is the only possibility that I can think of.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes