Same as a lot of folks. 1995 Pathfinder. Without warning will not reverse. Hardly any difference in RPM between park/Neutral/Reverse. How can I tell if I need a rebuild ?

If it is the automatic transmission and the other gears work but it wont drive in reverse. then I suspect the transmission fluid is too low.
are you checking the fluid correctly?
my sister had a car that reverse went out on. my dad asked her, how long has it been doing this?
she said 6 months and that she had to always park in a position that she wouldnt need reverse.
my dad asked how is the fluid? my sister said its always full on the dip stick.
she showed my dad it was full.
she didnt check the level correctly..
she checked the fluid when the car was cold. she checked the fluid when the car had been sitting for quite sometime and when it was not running.
.
run the engine till the transmission is warm. shift the car in all the gears. leave the car running in park on level ground and check the fluid then.

Guru is a ******. A tranny will not remain as the problem states the same rpm from R, P to N and not have any change at all when shifting.
There are a few problems but the most likely is the pressure is low meaning the tranny needs to be hauled out and the servo changed.