Also
Check the grub screw on the rear axle cvd area and center rear cvd area not just the big one that holds the pinion gear, but the cvd....
if that ain't it, then check the diff gear shafts inside the diffs, the ones the round gears capture that go through the outdrives.....the small ones...
if that ain't it, check the rear wheels, sometimes they spin on the hubs, I would put kyosho hubs on it in the rear....long story....
Hope this helps....