All I can hazard to guess is that you may not have had the 4wd fully engaged, causing it to "pop out" of 4wd. Had that happen in my dad's K5 Blazer long ago when stuck in a mud bog...or so he says. There is a shifter linkage adjustment you can check out:
http://www.autozone.com/addVehicleId...00c1528004c698
There is the possibility you may have damaged some of the internal gears, depending on how hard you were taxing the engine while pulling. If you haven't changed the transfer case oil yet, I'd do so and check the magnetic plug for metal pieces...maybe you'll be lucky and not see any, but fish a magnetic extraction tool in there just to be sure there aren't any pieces laying on the bottom of the case. If not, refill, reajust the linkage and I'd say you're lucky. Hope this helps!