My Jeep is the limited, with four wheel drive

My jeep is the limited, with four wheel drive. Its sapose to be all time four wheel, but today I was driving and the back end started to fish tail. If I have four wheel it should not do that right? And also when I go to put it in four low it makes a loud grinding noise. Im told it is the transfer case. Wondering how hard of a fix that is and how much it would run me? Would like some better input, if you have any other reasons my jeep would be doing this please let me know! Thank you for your time and efforts! Casey Blanton