Reverse Bounce
 

I have a 5 speed and while backing my trailer the truck will jump the entire time. Any ideas on what it could be?

Are the tyres bouncing or in the driveline.
Not enough weight on the tray
Broken engine/transmission mount.
Do you have a centre bearing in the driveline?

It's in the driveline for sure. There is a carrier bearing in the drive shaft but it seems to be in good shape. Mounts appear to be in working order. It's an 11,000 lb RV so the weight is there.

Have you used a bar or lever on the mounts? A broken one can look good and only spread when a load is put on it. (or try to jack the engine/trans up)
Have good look at the drivers side mounts.
Sounds like enough weight to hold the back down, guessing around 1500/2000 lb of that is on the truck.
Cheers Steve

Thanks for the comments. Had a new South Bend clutch installed and will see if this addresses the issue