T case knock need help!

[F2SMITTY]

So i put the new tires on my truck the other day and took her on her first wheeling expedition and when i got stuck in 2 wheel i locked her in 4 wheel i noticed a steady clunking sound. When i got back i checked out the hubs and they were fine, next differential that was fine, then i jacked it up and put it in gear and felt the t case knocking...before i dive into the case is there anything simple it could be is this common? it is only in 4 wheel. and does anyone know an estimate on how much it costs to have one rebuilt if worst comes to worst. Thanks