I have an '01 150 XLT, S/C, With auto trans, and 3.55 L/S diff. I had the rearend serviced at Dealership along with some other items at 53K miles.

The next week, I towed a 8' x 5' trailer with 1k lbs of furniture in it. Round trip was 16 hours total. At first, the rearend was whining at highway speed, and went away below 40 mph.

I took it back the following monday, and technician said it was ok, just normal noise. It started after the service. Also, I had to add L/S friction modifier, because the dealer said it wasn't needed unless rearend problems/wear were happening. I immediately questioned them on warranty book? I did not like their answer.

The next day, the rearend would make a loud grinding noise/feel when taking a Lt/Rt turn at light or stop sign. Up a hill in parking lot at work. It is really irritating me.

Any idea what it may be? L/S clutches?

I had it in the shop, a different dealer/state at 20 k for excessive metal in the diff fluid. Additive and fluid was topped off, and a new diff seal installed due to a leak.

I am taking it out of town, to a different dealer, who is supposedly a good dealer for maintenance.

Thanks in advance...