Honda 250 4 wheeler , rear end slips ... Help!?

86 Honda 250 4trax 4 wheeler .
Rear end slips and won't pull .I took it apart and nothing appears to be broke or wore .I've taken it apart numerous times thought I had it fixed put it back and same thing! When its on a jack it will turn the wheels but under a load u can hear the gears in the chunk slipping.gears look new.was going up a hill and it clanked and quit pulling... Thank u!!!