Kyle,
You mention only 2WD high. Does it pop out in 4WD high?
I had to go into my transfer case to replace some thrust washers. I also replaced the seals on the front shaft cap (around the shift rods) but didn't do anything else to that part. I think there's some poppets held in by plugs to keep the shift rods in place. When you're down there cleaning, you may want to check to ensure the plugs are in place. If one is missing, it may explain why the transfer won't stay in high.