F 150 making noise in reverse

Hello everyone. Thanks in advance. My 1997 F 150 makes a thumping noise when I accelerate in reverse (it goes away when I take my foot off the gas pedal). Universal joints or something else?

If you want to check for a bad U-joint, do this.

Raise the back of the truck until both wheels are off the ground. Place the transmission in neutral. Now have someone grab one of the rear tires and rock it back and forth while you watch the U-joints. If there’s a bad one, you’ll see the slop. This can also detect if there’s a problem in the rear diff.