I am getting a clunking and hard slam when going into reverse. Has only been happening when I first get into the truck.
Best pic I could get of the mount. U-joints don't cost much though I'd replace them at the same time. If I pull the drive shaft for the u- joints can I support the transmission from the rear where the drive shaft inserts rather than the pan? Pic of my pan shows it is not very flat. If I shouldn't then I'll need to built a frame that would use the entire pan for support. Assuming the pan is ok to use.

 

You should support the transmission at the rear and at the pan to be safe. I would replace the u joints before you replace the mount. The symptom you explained is common with bad u joints

 

Put a 2X4 across the pan where the magnet resides internally. I've supported the trans there a few times.

Be sure the 2X4 goes all the way across the pan though... end to end coverage.