hmmm, sounds like a reverse rev limiter vs a real disengagement...
When you hit the "disengage point"...if you keep on the gas will it rev up more?
oh yeah...would you say you are going fast? How fast are you going in reverse when this happens?
What makes you think it is disengaging from reverse? Does it coast? speed up? Clunk?
Sounds like the normal slack when you back up and ease off the gas and have the truck lurch a bit....
Solution....drive in D not R :D