Mild vibration at speed

[joeybsyc]

I originally thought it was a tire or 2 out of balance, but now I'm thinking it may be in the drivetrain... ( I had the rear tires static balanced (weights on rear lip only and the front ones spun balanced with stick on weights towards the center and lead weights on the rear lip) car runs perfectly around town, but take it on the highway, and going up a steady grade with my foot on the gas, it seems to have a mild vibration... not terrible, but feels like a tire out of balance...I can sort of feel it in the steering wheel but also in the seat of my pants... it tracks down the road straight when i take my hand off the wheel. The vibration doesn't seem to go away if i go faster, basically does it from about 60 mph up. Going down the same stretch of highway the opposide direction, with a slight grade DOWNHILL instead of up, the vibration is seemingly much less noticable. Any guesses on what may be the cause? Driveshaft out of balance? Universal? Something else?

One thing I didn't do and should have, was throw it out of gear to see if the vibration went away... I'll try that tomorrow if the weather holds out, but could use any ideas or suggestions on what to look for in the meantime... thanks in advance guys! -Jb