how do i know if i have a locker or not? I recently got a 2004 fx4 with 3.55ls but i noticed the front and rear diff had sealant squeezed out of the diff like both diffs had been serviced recently. I also noticed when i take a turn with a little gas i get a heavy vibration from the rear end. I'm not sure if the owner of the truck before me could have put in a locker in the back or not.

 

You do not have a locker. Your problem is that the rear needs serviced. It may need new clutches in the rear, but before doing that, have the fluid in the rear replaced with the proper weight (75w140) and make sure a bottle of the Ford friction modifier is put in. It would not hurt to have the front diff fluid replaced at the same time. It uses 75w90.
The shudder you feel is the clutches slipping erratically. If the clutches are still good, the fluid change with friction modifier should fix it, although it may take a couple of months of driving to know for sure.

 

The rear diff was probly serviced and modifer wasnt put in with the new fluid. Put in FORD MOTORCRAFT modifier and take it to a empty parkin lot and do some figure 8s to work the modifier in it might take 100 miles or so to fully fix it.

 

Thanks for the info, should i just take it into ford to have them do it? i don't have any warranty how much does this kind of thing cost? if i don't fix this will it do any damage to the rear end?

 

I would just get a bottle of modifier and add it to the current fluid. Its simple you just take out the fill plug on the front of the rear differential and add the modifier. If the fluid poors out of the fill plug just jack up the front of the truck and add the modifier.

 

Honestly, I would have it re-serviced, as you do not know what fluid was put in. It may have been the right one, or it may not have been correct.
Dealer can do it, but not sure of the cost.

 

Agreed. Id change it to but at the very least, id add the modifier to the old fluid. I wouldnt just leave it.

 

oh ok is there a specific friction modifier to get or is it all the same

 

Ive herd you can only use ford motorcraft friction modifier from the dealer. But my friend used the posi-trac modifier from advance in his bronco 2 that he had and it seemed to work fine.

 

The dealer is going to charge you about 150 bucks to change the rear fluid.

You should use 75w140 synthetic fluid and Ford modifier.