I'm pretty sure my rear prop shaft is out of balance. There's a rumbly vibration from under the back, especially at some speeds and on overrun. I pulled the tunnel cover and middle seat panel and drove around for a while. Its definitely from further back than the gearbox/xfer case area.

I know there are lots of things that can cause vibration, but there is a reason I suspect the prop shaft. When I bought this truck, it was delivered by the PO on a car dolly. He took the rear prop shaft apart and removed it from the diff when he towed it and replaced it when he got here.

I took it off at both ends and cleaned it up but don't see any sign of alignment marks anywhere. I've tried aligning the flanges both ways around, and also tried offsetting them 90 deg. each way. The vibration remains, though it is better or worse with some alignments.

I have pulled, pushed, yanked etc. checking for other things. I can't get any play in the U-joints or diff other than a small amount of lash in rotation.

Right now I have it back the way it was when I started. Short of trying every possible way of mating the 2 halves I don't know of any way for me to fix this. I'd like to avoid spending that amount of time if I can. Does anyone know of a shop in the Portland, ME area that can rebalance it for me? Any idea how expensive this sort of work might be?

Thanks