Transmission locks up when you let off the throttle?

Bought a 825i gator that would pop out of high range. Disassembled the transmission and replaced all the gears on the reduction shaft and syncros. Installed a new drive clutch and now when you run the rpms up with the machine in the air everything works fine. When you drive it and run it up as soon as you let off the throttle the transmission locks up. Any ideas guys?
Thanks