Drain your tcase fluid. Mine did that when I broke a snap ring. Slowly got worse untill it blew apart.
You found pieces of the snap ring in the transfer case fluid? It was just apart as well for pump rub.
Are you starting it, then as soon as it started, putting it in reverse?
It's a gmax kit with 1053 converter built or by diesel addiction.
It's like 95% of the time, reverse engages fine. Every once in a while it really hits hard.