Chevrolet: 99 Z28 Camaro Vibration, z28 camaro, rear wheels

Question
My camaro seems to vibrate when i reach about 50mph, and seems to run a little hard at 50mph and up.  I drive the freeway a lot to work so i average 70mph-90mph and thats when i feel the vibration most.  Wheels are balanced, fluids full, no leaks.  Any guesses?

Answer
Rob,
   I would start with the driveline. Physically inspect it to see if there is any obvious dents or if it's a little out. You can do this by lifting the rear wheels and putting a straight edge on it and rotating it to see if it's bent. When they are built, they are balanced, so check to see if there is any spot on the driveline where it looks like a small square weight was welded to it and has fallen off. Grab the driveline and try to shake it side to side and up and down to check if there is any movement that would indicate a u-joint going out. While you are shaking the driveline look at the pinion yolk and see if there is any movement or seepage around the seal that would indicate a bad bearing. Also at this time check the tailshaft on the tranny to see if there is any movement there. I would be willing to bet upon close inspection of the driveshaft and surrounding components you find your vibration. If there isn't anything wrong there, let me know and I'll dig into it a little more. Good Luck!