4 wheel drive problem

I have a 93, 3/4 ton. I haven't use the 4wd in a while. When I tried to shift into 4wd the lever moved but it didn't engage. I tried 4 low and 4 high repeatedly, but no luck. The next day, I tried 4 high again and nothing, but I was able to get it into 4 low with a lot of noise and grinding. It was tough getting it out of 4 low, but when I did, I tried 4 high and it worked perfectly. What's going on.