I am pretty sure it is the rear end of my 99 lariat F150 but not a 100% positive. when going in reverse and start to turn it makes a thud noise, when making left or right turns it goes thud but only between 0-25 mph. I noticed it once when going straight. It has a Limited slip rear end. Thanks for your help

 

I've had my '99 F-150 for 6 years. It's been doing it since day one. I also suspect something broken in the differential, because traction in snow or mud is crap, like an open rear end. Doesn't get any worse though.

Slightly possible that the yoke joint on the front end of the drive shaft needs grease, and the noise is telegraphed through the drive shaft to the pinion flange.

Dave