Rix06

Hello all, my name is Rick and I'm a newbie. I recently picked up an 06 Grand Cherokee Laredo with 122k, 4.7, and full time 4wd with low range. The other day when I drove over a fairly large crack in the road I heard a loud grinding sound and a vibration coming from the t-case area. It stops after I put it in reverse and then back to drive. It has happened a few times since then but I am not sure what the cause is. I would appreciate any input or advice you guys can offer.