The dealer replaced the ring and pinion gears in mine twice and that did not fix the problem. They put a brand new whole rearend that is the only way to fix this problem. They had to go thru the ford rep to get this done IT IS THE ONLY REAL FIX! Ford is aware of this major problem so they should take care of it for you after they try the ring and pinion gear. Lariat 3.73 LS 5.4 4x2 Screw , Tow Pkg
good luck