My 89 XJ makes a ZZZZZZZZ soound when driving. It pops out of gear. I can pull over and use the selector to put it in Park then back to Drive and it is ok until I hit another bump. It will not "pop out" when I am running in 4 LO. I have adjusted the linkage on the T-case but it keeps popping. Not sure if it is a T-case problem or the linkage is worn out. Any suggestions???