put it in reverse it slipped

This is the situation I had some bad motor mount I replaced them I went to the store came out started my tribute put it in reverse it slipped then caught gear I brought it home someone told Me I had a bad CV axle just pull it out and my brother-in-law came over and said don't do that put it back together won't find out what's wrong put it back together so it up and it drove just fine no problems my wife and I got into it then we're going to leave and put it in reverse and it done the same thing it slipped and cut it into gear I got out and hook the battery hooked up just fine could that be a sensor causing the problem