Does 4high work like it should? If it doesn't, I would suspect that something is wrong with your front hubs. And just to make sure you know this, to engage 4low you have to have your foot on the brake and the truck in neutral, or if it is a manual you have to have the clutch pushed in I believe. If 4high works but 4low doesn't, I would suspect something in the transfer case broke. Also, if you do not have a limited slip in the rear, it is perfectly normal for just one tire to spin.