97 Tacoma with auto transmission loses Power when in reverse

I have a 97 Tacoma 4X4, V6, Automatic. Here's a quick explanation of the problem I'm having...when the Tacoma is in reverse it loses power. I would explain it as being very similar to the safety feature on a Polaris 4 wheeler when it's in reverse, they lose power until you push the yellow button that overrides the safety feature. It cuts out terribly and won't even move up a steep hill in reverse. When it's in any of the forward gears it runs and operates perfectly with zero issues. I could burn the tires off to the rims it has so much power in forward gears. Again, my problem only presents itself when I'm in reverse trying to back up with a load on the engine. Please help as my mechanic had it all last week and is ready to give up. BTW- I live in a small town and there's no foreign car specialist here.

 

I would be looking at the transfer case and rear brakes .
If your mechanic had it for a week , what did he try in an attempt to fix the problem ?
Did you get the truck stuck and try rocking it out at anytime before this happened ?
Did the loss of power happen all at once or has it slowly got worse ?