Four Hi/Lo locks up

That's odd that it won't move at all. I've had those transfer cases apart and there's a solid shaft that runs through the t-case for the rear wheels and a cluch pack driven by a chain turns the front driveshaft. I would guess the problem is likely in your front end if it only happens in 4x4. I need more info, does it happen in 2wd at all? Best check ALL your fluid levels (front diff (75W90), t-case (GM autotrac fluid), and rear diff (75W90), but at this point, it's probably useless because it sounds like something is majorly f***ed up.