4x4 not working

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I have a 1987 silverado 1/2 ton 4x4 and put it in 4x4 this morning due to icy snowy conditions. I made a couple of turns and it didnt feel like i was in 4 hi. Then i got ready to go up a dip to cross the train tracks and the back tires would do nothing but sping. No effort from.the front tires. I even put it kn 4 low and saw no results. Its a stock transfer case as far as i know, but i dont even know where to start on 4x4. The shifter goes into position fine and i even felt a little grind when i wasnt paying attention and didnt have it in nuetral while putting it in 4 lo. Anyone have an idea on where i need to start and look for? Thanks