I have an 01 super crew with the 4x4 switch on the dash. When I put it in 4hi while moving, it grinds even if I'm not on the gas, or coasting in neutral. If I'm stopped it does not grind when I engage the 4x4.

 

4x4 will always make weird noises.

Make sure your transfer case and differentials aren't empty and have new fluid.

Other than that not much you can do until it stops working. Make sure you're engaging on a surface that allows your wheels to slip.

 

I'll have to check the t-case, but the fluid was changed last summer in the front and rear differentials