Differential
 

I have a 2002 f150 xlt and the rear end feels like it is possy locked is that my rear end going bad? i have been told it was the rear end but i have also been told that it was my U joints? Can anyone give me suggestions?????

You will have better luck if you post this in the 97-03 section of the forum.

And what the heck is possy locked ?

I moved you into the correct forum.

We need more details to help you here. First is it 4x4 or RWD? What engine is in it? What is the axle code from the door jam sticker?

Positrac is a GM version of a Limited Slip differential so technically you won't have positrack unless someone changed the rear end.

What exactly is the truck doing? Are you having issues going around corners? Is only one tire slipping when in poor traction or are both wheels slipping?

I have a 2003 RWD f150 reg cab, 113K miles, 5 spd manual. Today the rear end made a big "bang" when I left the driveway, shifting into 2nd. I put it in 'R' backed up a bit and limped back into driveway, about 100'. The left rear tire locked up the last few feet. I bought at an auction August 2016 and it's been fine. Any ideas.??

Elricfate how long you been a mod?

2 days

IF it locked up you probably broke a major part. It could be the driveline, or ujoint, but it could be a possible carrier pin, ring gear, pinion, something with the carrier/ring gear. Pull the rear cover and see how many pieces are in the oil, or if there is oil. You did say you bought it at an auction. You'll need the axle code, which is on the door sticker on the drivers side to see what you have. It shows the word axle and below it will be the code.

in this case i would actually see if the diff still has the identification plate, to make sure it matches what the door jamb sticker says. if he bought it at an auction, you never know what has been swapped or replaced especially since its an older truck.