A few things I would check:
1. Slipper clutch. Make sure it is tight, not immovable; to where you can just barely turn it with your fingers.
2. If tightening the slipper does not work, take the engine out and make sure that your clutch is working, and that the flywheel is securely fastened to the engine. They are not positively attached to the engine; they are squeezed onto an angled collet and friction spins the flywheel. If the nut is loose, the flywheel won't turn.
3. If your slipper and clutch/flywheel are all OK, then check your drive line. Something in the transmission or diffs could be binding or broken. Also make sure that the drive cups connected to the input/output shafts on the transmission and diffs are all securely fastened and that all of the drive shafts are OK.