HI everyone, Happy New Year. So I have recently purchased a 1988 GMC pick up truck 4x4. I dont think the 4-wheel drive is working. When I shift into 4x4 low, I sometimes get a grinding noise. If I put it into neutral first and then shift it may not happen. Usually doesn't but I have heard it occasionally. When it does go into 4x4 low or high, it just doesn't seem to have the torque that I expect it should. How do I test it?

thanks for any suggestions.