Alright so i have a 97 suburban with 215k miles and im having reverse problems.. when trying you give the truck gas or try backing up hills or putting any "load" on per say the truck will pop out of reverse and make a terrible grinding noise and you have to put the truck back into park to stop the grinding them put back into reverse to attempt to back up. All forward gears work properly and the truck drives just fine except when trying to use reverse. the truck is 4x4 with transfer case NP243c and has a 4l60e with a beast sunshell gear installed, also rear end was replaced approximately 3k miles ago but grinding noise also would occur before the swap.

 

I may not have the best answer for you, but here are a couple thoughts. I think that trans has a linkage that manually changes gears from gearshift. I would see if there is any adjustment there in the event it is not seating all the way in R. The other option is to change the fluid and filter. Although problems would usually show in the other gears. If none of the above fixes it, it may be internal and therefore a bit more complex.