ZDriver.com - Steering wheel vibrating/shaking above 60mph+ after lowering suspension

Hey guys! Ok, I've got an issue with my 91 300zx that no one has been able to find a solution to. I replaced the struts with new coilovers 4-5 months ago, everything was fine, except the mechanic left the car riding too high. So I went back and they lowered the car to the original ride hight that it was at with the struts and now whenever I get on the highway going 60mph+ my steering wheel starts vibrating. The steering wheel has never shook before, only after I took the car back to be lowered with the new coilovers on. It was perfectly fine with the new coilovers and riding too high...but once I lowered it, vibration in the steering wheel started. I've had my car balanced and aligned more times than I can count, at multiple places, and it changes nothing. I've replaced the tires, brake pads and rotors (front rotors have now been replaced twice in a matter of 4 months because one was warped for some unknown reason) and none of this has changed the vibration in the steering wheel. I don't know if it's something with the U-joints or something not sitting right with the suspension when it was lowered or what's going on. I took my wheels off and put the stock wheels back on and there was still vibration in the steering wheel, so I know it's not the wheels/rotors/brakes. Any suggestions?? I've got a new mechanic helping me to try and figure this out, but for the sake of not spending more money, I thought I might post on here and try to see if anyone else has had a similar issue or another thought as to what this might be? The only other thing I've noticed is that when I'm in 4th gear going 60mph+ the vibration is very minimal, but as soon as I put it into 5th gear, the vibration gets worse. Help! Thank you in advance.