Mule 2510 diesel, pops out of gear when decelerating
 

When in driving in High, particularly going downhill, once I lift my foot off the pedal there's a whirring sound and it won't go back into gear, forward or reverse.

I can shift it into Low and get going, and sometimes "speedshift" from Low to High and it will work.

Please tell me this is a simple fix. Kawasaki mechanics around here are pirates.

Thanks