2006 f150 rear end clunking
 

I have a 2006 f150. When I start out turning you can feel something clunking in the rear end. I took differental cover off. Fluid looks good. All the gears look good. I put truck up on jack stands. With truck in park. I have a hard time turning a wheel. When in neutral, the wheel turns easy and the differental gears don't move at all. The ring is stamped 373. Any suggestions?

You didn't state whether it's a 4X4 . If not then you could have a posi rear that requires an additive ( friction modifier) to be added to the differential lubricant. Just get some from Ford , add it and see if the problem goes away.

It is 4 x 4.

Did you check the driveline u-joints?

Yes, I did. I pulled the gears out of the rear end. I put a axel in the gear box and I can not turn it by hand. The spider gears will not turn. I think they did not have the additive in it for the clutch plated. All gears look good. Now should I replace the clutch pack or just put it back together and put new lube with the additive in it?

If you can't see any apparent damage or anything unusual I would put it back together with the proper lubricants and run it on the stands with the wheels on and give the lube a chance to get everywhere it needs to go. Do not exceed 20 MPH on the speedometer. Then engine off in neutral try to turn any one of the axles. It should be easier to turn than not at all.