Weekender

This symptom you've described sounds exactly like what I've been trying to put my finger on in an '07 FX2 I'm looking to buy. The truck sits on their lot and probably hasn't been driven much since they got it. The two times I've had it out, I notice what might be a dry slip-yoke clunk when getting underway, but the odd vibration you describe comes and goes during the first couple of miles, then diminishes after about 5 or so before I get it back. Doing a tight u-turn through a gas station wasn't smooth and it felt like the rear axle was winding up.

I'll have it at my independent mechanic this week if work allows and will ask them their opinion.

How do you check for this without opening the rear end? I know the guys in the shop and they're doing an inspection for me, no charge. I can get under the truck with them while it's on the lift and run through my list of stuff. If I don't see obvious U-joint or other shaft issues, I may have to pay them to open the diff, but it's not my truck.