I did a search and read the topics on this but didn't find the answer I was looking for. I bought a used 2001 f150 screw, love the truck but I'm getting the rear end chatter. I've changed the fluid in the transfer case now I want to change it in the rear end.

I've looked at the tag on the rear housing and don't see a L on the tag, so I assume it doesn't have the limited slip rear end, but I notice that the cover has been removed before. Now the guy that had it befor me put a 6 inch fab tech lift on it, I'm going to assume he changed the gears after the lift, so would it hurt if i put the additive in it just incase there is now a limited slip rear in it?

 

Just because he had a 6in lift don't mean he re-geared...he could of ran 35,s with no problem.

 

according to the sticker inside my door, the truck has 3.73 limited slip gears. Being that I know nothing about this sort of thing does that sound ok with the lift?

 

what size tires do you have?

 

It got 315/75 r16s on it now, going to need new tires this winter they are getting wore out.

 

3.73s are fine ,your rear end should take 75w140 full synthetic with a friction modifier or you could run royal purple by it self

 

thanks guys, going to try to get that done today to get rid of that damn noise.

 

I changed mine about a month ago and the noise and vibration went away. Just make sure you were gloves because its hard to get the smell of diff fluid out of your hands. I put in Motocraft 75w140 and used Motocraft friction modifier.

 

is that true... you dont have to add the additive with royal purple...im gettinready to change my fluid in my 08... and ive always used royal purple on my oil changes.... im going to use it for my rear gear oil ..

 

I didn't have time to change the fluid yesterday, after fighting for about a hour to get the fill plug out I had enough for the day. It was low so I added some royal purple to get me by till next weekend. You are right about the gloves, I stuck my finger in there to check the fluid level and I never thought the smell was going to go away.