Even back in the army days, sometimes the shifter linkage wasn't adjusted correctly after a changeout ... I had situations where the lever had to slap the seat frame hard to get it into Hi range, other times the seat frame wouldn't let the lever travel far enough, and other times when tight to the seat frame the transfer would pop out into neutral the second you put considerable torque on the engine.. In Hi, the tip of lever should point between 10 and 11-o'clock and the lever shaft very close to touching the seat frame.
I have also seen the adjustment too far the other way, where the floor pan won't let the lever go into Lo.. Or it does but only engages the Lo gear halfway, popping out of gear the second you apply considerable torque. Had that happen on a truck the day I bought it and was well-into a very steep hill with a car tight on my butt.. Fun surprise. There definitely is a sweet spot for proper linkage adjustment on these things.
What others have mentioned is correct, and first get that linkage lubed up... It doesn't hurt to squirt some oil on that shift rod poking out of the transfer case while you're under there .. You should pop off your floor pan, it allows better access to lube the lever pivot point and observe what's happening as you move the lever.. Hopefully nothing's wrong with the case