Maybe you have big rock stuck in Diff.
Maybe rock in belt, or maybe roll backwards, then better.

LOL

Just messing,

Check the belts etc. for debris, and SWEEP that parking lot before you run.
Also check your clutch. If you cant resolve by troubleshooting, take the whole thing apart and put it back together, replacing any worn parts.
You will certainly stumble onto the problem as you put it back together.


Dbow