The transfer case shift motor is working. Otherwise you would not have gotten into the "lower" gearing you described. The low range is a second set of gears in the transfer case. The transfer case can still be in low range and if the hubs or anything in the front driveline is broken you will not have 4WD.
It is possible that your auto hubs are broken, as it is a very common problem. However, I would not jump the gun and replace them without checking everything else out first. When taken care of the auto hubs usually work well.
I would put the truck in 2WD and spin the front driveshaft with your hand. You can reach it from between the driverside front tire and fender. If it turns freely listen for clicking. If it turns a little and then stops it would appear the hubs still work....some... and they need taken apart and cleaned.
While you can easily go out and buy a manual transfer case and hubs, until you verify the problem your just throwing money at it.