'haunted' transmission. NV 4500

[JEdwards]

My truck is 1999 2500 4x4 stick shift. haven't driven it much this last year, cause life happens.
the other day, after a 40 mile run, I parked on level ground, engine running, in neutral, no brakes. the truck would make a noise,sort of a 'clunk', then actually 'move' a few inches. it was like it was trying to slip into gear. but I know that is not possible. at first thought it was a fuel 'surge' issue, but my go to guy, says transmission.
I had the transmission flushed, and new fluid put in. but no change.

any thoughts before I hand it over a shop????

 

[NIsaacs]

What did the fluid look like, was it low, contaminated with metal flakes, grime or discolored?

Sounds like the pocket bearing is bad and trying to seize. It is located between both input and main/output shafts. If it tries to seize it will cause the input shaft to try and turn the output shaft.