Originally Posted by jiminpc
I started noticing a vibration at slow turning or starting out from a stop into a turn. My 2003 F150 only had 22000 miles when this began and has become incresingly worse. The dealer told me that the only fix was to replace the clutches in the rear end. $400 to $500 ouch! I tried adding a little extra conditioner with little or no effect?
Take the back cover off. Inspect the gears and such. Look for alot of metal coming out of the fluid. If all looks well then clean it all up really really good and refill the diff with 75w90 synthetic and 2 tubes of friction modifier..not necessaraly in that order. Use a GOOD rtv to reseal the back cover. I like the wurth but thats hard to find. The black in the areosol cans(aerosol like cheese spread can) That works the best for me. Makse sure there is NO oil on the sealing surface. Use copious amounts of brake clean on it and scrape it GOOD!!
If you still have problems after that then change the clutches. If cost is an issue it won't hurt to try this rather than pay that much for a rebuild.