Well, usually the problem is with the 2WD/4WD shift hub not releasing the 4WD Idler Gear, as you can have different loads on the front and rear axles which can put a binding force onto that shift hub.
So typically, if you are wheeling and are under a load, like going down hill or turning a very sharp corner, it is difficult to get OUT of 4WD and into 2WD.
But if you are having a tough time going from 2WD to 4WD, then this is highly uncommon.
It is impossible for the interlock pin to "lock" the t/case out of 4WD, so it can't be that. My only guess would be that either your shift rails have excessive dirt or rust in there, the shift springs are rusted and now have a spring constant approaching infinity, or your 2WD/4WD shift hub is badly worn from many miles of continuous grinding into gear, and it will not go over the 4WD idler gear.
My advise to you would be to first pull out a 6mm allen socket and remove both of the shift spring retaining bolts located at the top of the #2 t/case housing on both sides (drivers & passengers) of the t/case (they are the only allen head bolts on an entire stock t/case). Then remove the shift spring and the 0.5" detent ball for an inspection. If they look good, then I would squirt some WD40 in the holes while someone else wiggles the shifter around trying to engage 4WD.
Best of luck
BigMike
