Help! I’m not sure what to do, mechanic told me driving rack, I don’t think... someone else told me brake calipers. How to pin point the problem?!! Thx

 

Why is this happening

 

If your wheels cease to rotate when reversing, likely it's related to your brakes. I'd service them just to eliminate the possiblity and make sure the moving parts are lubricated properly, such as the slide pins on the calipers. Also make sure your calipers themselves are in good condition, they may need to be rebuilt/replaced depending on how many miles on the rig. I'd also flush the brake fluid and when you install everything make sure to use the "M" spring clips that spread apart the brake pads when you're not braking.

Never heard of the steering rack causing a tire to not rotate, dunno how it's even possible as the steering parts are independent of the rotating assembly at the wheels.

 

Loose brake pads? Don't know how steering rack would come into play in this situation.
I would start with making sure wheels are not installed upside down.