My 1996 f150 5.0 4x4 M5R2 with 1356 manual shift transfer case is making a loud slapping clunk. It's very noticeable when shifting gears even in 4x2. I have it all out to redo the transmission. With it in 4x4 on the ground turning the front driveshaft yolk back and forth makes it slap hard.

I was thinking stretched chain but wonder if could be something else.

other than the initial slap it turns smooth.

I've read through this thread from another site which shows a tear down and rebuild. https://www.fullsizebronco.com/threads/rebuild-your-own-transfer-case-bw1356.135334/

I will eventually take it apart and see what's going on but wonder if anyone has a starting point.

Added a video of it on the ground making the noise.