1) What about the 2 front driveshafts and the center driveshaft? Are all of them straight and not bent? I know that center driveshafts on 4x4 models are prone to bending.
2) Try to move/feel the front and rear diff with moving that pair of wheels. Do you feel like anything might be binding or have resistance?
3) Does your slash have a center diff or a center slipper clutch?
4) Are all of the diffs in the car properly greased or filled with oil?
5) More of a wrenching/tear down part, are all of the bearings in the car clean, greased and roll freely and smoothly? Especially the bearings for each diff in the gear boxes and the diff pinion gear on the gear boxes