Well I had my truck at the tranny shop for a tranny flush and filter and the service tech said he noticed on the test drive that the rear end seemed to be acting like it was a posi unit (which it is) with no friction modifier in it. My question is could it of come from the factory like this I've owned if from new an it has 100,000k on it and I've never changed the rear end fluid because the owners manual says it's synthetic and it never needs to be changed unless it's submersed in water which it hasn't. Also today I bought a bottle of friction modifier from ford $2 and was planing on just adding this to the existing oil. Is this a good idea or could it cause any problems.

thanks for any advice

ps 98 f150 sc 4.6l 4X4

 

I think the service tech was blowin' smoke. To test drive a vehicle and tell you that makes me believe they also wanted to change out your diff fluid since they had also changed the tranny fluid. The clutch pack in the LS diff can wear out but I doubt the test drive would tell him that unless he tried to lay down a patch of rummer.

 

i don't think the service tech was saying that the rear end grease needed to be changed. he just made an observation. you can definitely tell when a posi rear end needs some friction modifier added. it will hop/vibrate when you start from a complete stop while on a turn. or it will chatter or groan when you go around corners. my '02 supercrew with only 5k miles is starting to do this, i'm gonna have the friction modifier added at the next oil change. i suspect the modifier was not put in at the factory...