2006 F150; Upon first accelerating forward there is a telltale thump somewhere in the drivetrain. If moving forward slow enough there is no problem. There is no vibration which would indicate a u-joint is bad. Auto trans with 4.6 engine.

Any ideas?

 

Does it also do it when slowly coming to a stop? I would feel a thud every time I accelerated, almost like a hard early shift from 1st to 2nd. Did it when decelerating as well, similar to a hard downshift.

This took 30-45 minutes and solved my problem.

http://www.revbase.com/BBBMotor/TSb/...dPdf?id=177141
http://www.ebay.com/itm/Genuine-Kit-...1YZsjS&vxp=mtr

I have the 2-piece driveshaft on my 2010. Not sure what the 2006 has, but it's pretty much the same thing for a 1-piece. You would just grease the spline where it goes into the transmission/transfer case.