you might of engaged the 4WD at to high of speed.
Have you had the override button overridden? that could be the problem. Then again, it might be time for new hubs.

 

Ok Here You go. Tourqe spindle nut to 160 to 170"lbs while continously rotating the hub. Step #2 Back of nut 1/5 turn. Step #3 Rotate axel several revolutions by raising the rear of the machine and rotating the rear wheels with the machine in gear. Step #4 Re-tourqe the hub nut to 108 to 144" lbs then install cotter pin if the pin does not go in tighten the nut slightly until the pin goes in. DO NOT GO OVER 144" LBS Refill hub after putting on cover with type F tranny fluid or hub fluid. DONE