hey guys-

according to my door jamb i have a 3.55 limited-slip differential (axle code H9) but its not engaging. on ice or mud only one tire will spin like crazy. the truck has 136,500 on her and ive only owned it for about a year now. do you think it would be a good idea to go ahead and change the diff oil? if so, what would be the best procedure??

 

Understand limited slip doesn't lock all time on ice or mud. If one wheel has no resistance, it will spin and the other won't move.
http://en.wikipedia.org/wiki/Limited_slip_differential

Still not a bad idea to change it if you feel the need, especially if as been submerged.

 

I would change it.I'll bet you'll find it milky and it does affect locking diffs more than non-locking. It's really important to put a full bottle of friction modifier in a locking diff. and it calls for synthetic 75W140 oil in the rear. I wouldn't substitute anything else.

 

In case you need it, here's a helpful link that I found on how to do change the rear diff fluid.

http://www.fordf150.net/howto/differentialfluid.php