When I put my XJ into reverse and back up there a sudden loud droning/whining sound. Is it the rear brakes? Sounds like it but I can't tell.
When I go forward it never happens.
Thanks...

 

stick or auto matic

 

whining is pretty normal in reverse especially if you have a manual. I suggest you back up slower.

 

It's an automatic 6 cyl.
Even when I back up slowly it happens.

 

does it happen only when the truck is moving? or does it happen in reverse and with foot on the brakes also?

 

Happens only when moving. When stopped and foot is on the brake, nothing.
Take my foot off the brake and go slow in reverse, there's the horrible sound.
All gears are fine except reverse.

 

is it more of a grinding sound? It's likely is your rear brakes dragging - they may have been improperly installed.

 

All gears are fine except reverse.

Sounds like you answered your own question !

 

reverse uses a straight cut gear, which produces a whine, while most gears nowadays are helical cut, which cuts down on whine

helicals are weaker, but i guess people really dont like the whine

I have a Muncie M-20 with 1st through 3rd have straight cut out of a M-22

sounds intense

 

Sounded like it was coming from a rear brake.
Checked them out and the brake pads and everything look old as hell.
Still a little brake pad left and no leaking that I can see.
I think one of them is locking up in reverse.
It's a grinding high sort of whine. We'll see.