1999 SUB 4WD Grinding Sound

[dlcooldude]

Have a 1999 K1500 SUB with the push button switches to change the drive from 2WD to 4WD, etc. Son drove truck on weekend and he put it in Auto 4WD and after 20 miles he heard a low grinding sound and then the wheels locked up. He switched it to 2WD and drove 100 miles home without any issues. I tried to switch to 4WD HI last night and the sound came back; then when I switched it back to 2WD the sound is still there. The dash light at the switches is not on when I push the button.
Any thoughts as to the problem???