I did take it to the power washer and spray them off. But I will take off the wheels and inspect.

This is actually kind of what it feels like. And the first time I noticed it (forgot to mention this) I had been stopped on a hill for a few minutes in park with the e-brake engaged. Took the e-brake off and backed up, that's the first time it bound.

 

I'll put it on jack stands and spin the wheels, if I can't isolate anything I'll take it to the tire shop and see if they'll let me use their lift.

 

Sounds like you may have either a CDL failure or a front or rear diff internal bearings may have failed. Have you checked your propshfts for play? Up down, side to side?

 

If it was this it would happen in drive as well. I'm trying to think of things that are affected by the rotation of drive train components..

 

Hummm......a brake shoe that has disengaged on one end....drags fine in forward motion...then when in reverse, it jams and tries to bind into the drum????

 

This may be possible - pull the drum off and take a look see

 

On AWD these are rotational drivetrain components, happens on older Audi's. Brake shoe is definitely a possibility too!

 

The only problem with putting it on stands or up in the are is that it takes all the resistance of the tires being on the ground out of the picture.. Were you able to replicate it while on jack stands by putting it in reverse and hitting the brakes?

this is true but pinion bearing issues arent going to change in drive or reverse because nothing is really changing when torque is applied the opposite direction. Where if a bad ujoint is being pushed instead of pulled because of how the drive shaft is rotating and the brakes are applied it could bind...

Another question.. Have you checked your engine/transfer case mounts? I am thinking going in reverse and braking may be lifting your tcase off the frame?

I'm just trying to come up with answers to both issues - reverse and happens when brakes are applied.. This kinda rules out the e-brake drum/shoe answer..

 

Mounts were my next guess as well.

 

Broken caliper bolt, caliper moving.