The actuator on the front axle is a pretty common failure point. If it's a manual shift transfer case it is pretty rare for something to happen internally to the case that causes it to not go into 4wd, especially without a bunch of other issues, noises, and clunks. I have had the transfer case shifter linkage actually rust out and become unattached the case, but it was obvious something happened because the shift lever went completely limp.
You can do some very easy diagnostics. In 2wd with the truck parked see if the front driveshaft will spin freely. Then put it in 4wd and see happens.