2006 4WD Issue

My 06 avalanche sometimes makes a "grinding" noise when I go from 2 to 4 wheel drive. I hit the gas and it sounds like it doesn't lock into place as the truck cannot move when it happens, just makes noise. Any ideas?

Also, anyone have any idea why it doesn't go into 4 wheel low by chance? Light just flashes and doesn't go into low mode.

Thanks

Is it still under warranty?
If so take it back to the dealer.
How many miles on it?
It will need to be put on a Transfer case capable scanner to get codes and to check it out.
Post back any codes.
MT