I am not familar with the CRT.5 and how the drives are attached, but I've seen more than one get fooled by a missing drive pin that connects diff and the center drive. Tmaxxes are notorious for wasting diffs and most automatically assume its something in the diff (spyders wasted or R&P stripped) but get fooled when they take it apart and realize the drive pin came loose and got spit out without hitting anything and thus instant 2WD.

It would bother me that I couldn't find anything wrong. Gear diffs are just not that complicated. Are diff cases and supports weak enough that they can flex under torque and let R&P come out of mesh? Flexing is what kills Tmaxx R&P and why most run metal diff cases - especially if you're not running stock power plant.