My friends Blazer has a 203 with a part time shaft kit installed and had been working fine, now when we go wheeling and the transfer is shifted to 4hi or 4low it works great and then with no grinding or no noise at all it slips out of 4 wd. We opened it up and everything looks good. We even adjusted the shifter. Could the nut on the rear output have been tooo loose?? He had already taken the back cover off when I got there. Any ideas???