When at a complete stop, then pressing on the gas pedal, it sounds like a single clunk from the back, then no more noise.
If I ease the gas slowly, it doesn't make the clunk.
Anyone have an idea what it would be?

 

Search this forum for F150 driveshaft spline. XG-8 PTFE grease to remedy, if that’s your problem.

 

There's a video for that.....

 

Slip yoke needs greased. Had that problem with my 2023 last year. I bought the special grease, took the driveshaft out and lubed up the yoke and reassembled. Problem solved!