harmonic vibration

I have a vibration inside my car that seems to be coming from the drivetrain , its not real bad but you can feel it in the seat and the shifter. It seems also to be worse on county roads and better on state highways, at 60+ mph. I jacket the car up and put it on jackstands and started the engine reving it up to abour 3000 rpm. it didnt seem to vibrate I then put the car in gear ,brought the speedometer up to 60+ mph and the cab,seat and shifter have a definite harmonic vibration that is annoying the heck out of me. the tires have been balanced but I dont know if the driveshaft has or needs to be. the transmission is a C4 and the rear end is an 8.8 ford. The driveshaft was proably a custom because the car is a 34 kit car I bought half finished from a gentelman who ran out of money.:(

make sure the pinion angle is correct .. my mustang does that also since i narrowed the rear .. something iv'e got to fix this winter .. the pinion angle should be the same as the transmission angle.. it`s easy to get a bad alignment with custom stuff

thanks for addressing my problem with the vibration. I did some more digging in this club forum decided to take the drive shaft out and replace Ujoints but thay looked and felt new so I took it to a driveshaft shop.Turns out it was an out of balance drive shaft 10 grams on the front and 12 grams on the back of the drive shaft. dosent sound like much but what a difference I installed the driveshaft took the car out and got up to 98mph before I ran out of road and it is smooth as silk. yeha!!

you got a good enuff road in georgia to go 98 " wow" .. ya`ll must have asphalt ..we still have mostly dirt roads in alabamer .. cant get a lottery voted in .. ..:p