i have a 86 Toyota pickup and I was out playing in the mud in 4 wheel drive and my transfer case popped and started slapping and threw the transfer case out of gear and when I put it back in gear everytime I try to drive it pops right back out of gear except when I put it in reverse what could my issue be?

 

Is it the stock case or an aftermarket? Stock gears are suppose to be cut to help hold in gear but some aftermarket cases don't use the same design.

Regardless I'd pull the shifter and inspect/replace the detent ball and spring. Make sure the interlock pin is intact.