My NV5600 jumps out of 6th gear on sudden decelleration, such as cancelling cruise or letting off pedal suddenly.

It doesn't make any unusual noises, and shifts ok.

 

sounds like syncros are going out.

 

Shouldn't be synchros, detents are more likely. They shouldn't be hard to fix up but I don't know the NV very well. A lot of trannies can be done in place with plugs on the outside or in the shift tower.

Do you drive with your hand on the shifter? Quick way to wear detents out. If not, then they could be a sign of more general wear items coming up soon, like synchros, throw bearings, clutch etc.

Cya

 

Knocking it out of gear without clutching will cause that over time