i bought a 91 chevy 2wd truck from my cousin about 2 years ago. he had drivin it for 3 years, i drove it 6 months after i bought it until i decided to swap the tranny with my roadmaster wagon. when i pulled the trucks tranny, the whole center was busted out of the flexplate. it drove excellent, never made a noise, vibrated, or anything. all of the broken edges were well rusted with pits. i have seen busted flexplates in the past but a vibration or noise lead me to them.
The verdict is in... I have no idea what it was. It was doing it extremely consistently on the drive home. Got some tags, replaced the drag link and it didn't do it again. Had it around 3-3.5k pushing 85 down i70 and it was nice and smooth. Only thing I can think of is that it was the rotten drag link that was doing it. Either way, she's good to go and cruisin'. New shocks tomorrow morning and new brakes when I scratch some cash together. Thanks to everyone for your input and hopefully it wont start doing it again.
that shaking syndrome. Used to have a Plymouth Reliant wagon with that problem. The mechanic we took it to because Midas couldn't figure it out said, it was the fly wheel with cracks around the holes where the bolts go through it, to hold it in place.
I was gonna say steering or suspension. The guy did say that Dad did a driveway burnout and nothing. That would have surely loaded up the drivetrain! I had a Bug that would vibrate so bad you couldn't steer it sometimes. Would happen around 40mph. There's a skinny "shock cylinder" in there, bolted to the body and to the steering links, replaced that...whole new car.