I recently bought a 2004 F150 and it had the awesome slip yoke clunk. After getting the repair kit from Ford and dropping the rear piece of my 2 part shaft, I lubed up the yoke and reinstalled. Unfortunately, I had also had an alignment done and a tire rotation (new tires anyway). Now here's my problem - when I go 65+ I get a vibration and a deep bass sound. (First time on the highway since alignment and slip yoke lube).

I took the truck to get the tires balanced as my first thought was that when the tires were rotated, the fronts and backs wore a little off...however, balance was done and I rotated the tires back to original. This didn't make a difference.

So then I thought maybe I lined the shaft to the differential flange wrong, so I took the shaft off and flipped it 180 degrees. This has seemed to cut out almost all of the vibration, but the deep bass hum is still there at 65+.

Any thoughts? I am at the point where I am looking into new driveshafts, but this looks like it's going to cost from 600-700 for a 2 piece shaft that is decent. Just would suck for the shaft not to be the issue - not sure how it would have been thrown off balance?

Any help or thoughts would be appreciated!