My transfer case keeps popping out of high gear it has no problem
in low gear any ideas?

Check the linkage. It could need adjustment or replacement.

Do you have a body lift ? If so the rubber boot could be pulling the shifter out of gear. Does it only come out when you let off the gas ? It doesn't take much pressure to make it pop out.

My D18 would do that, well just fall out never pop. I acounted mine for being 35 years old, and just worn out. Fixed with bunji(sp) cord. However check everything else first.