Hello to all...
My 97' F150 4x4 w/Off Road Pkge has been making a sound (not evrytime) when I put the truck in reverse, I know what U-joints sound like but today when I was backing up, it actually felt like it slipped (could be wrong) with a double/metal cling! I have searched through the forums, there are similar situations but none like mine (slipping). My truck has 126K and I have owned it for 3 yrs now. Planning on taking it to the dealership because I have a lengthy trip coming up...don't wanna lose the rear-end. Any help is greatly appreciated!

 

U-joints are the most likely culprit, however, it sounds like you're familiar with their failure mode noises.

I remember a member on another forum talking about ensuring proper levels of limited-slip additive. He shared a similar experience to yours and said that after putting the additive in and working it in by driving in figure8's for several loops, the slippage problem disappeared.
f150forum dot com/showthread.php?t=645&highlight=rear+differential+s lip

Or I suppose you could have an issue with the rear diffy carrier loosening - not at all common, but I suppose, possible.

 

thanks for that wde3477...I did 4-get to mention that I notice (sometimes) that when I'm cruzin about 50 ish and I let off the gas and start to slow...I here like a "plastic" grinding comming from about the middle of the ruck. Does this truck have a speedometer cable...if so, what do yo think the possibility of that making the noise is?