I have a vibration if I floor my car while cruising at highway speeds. Normal cruising is fine its only when it gets that hard downshift and starts pulling. I suspect its pinion angle since its lowered a lot on coil overs and I don't have an adjustable torque arm. I have UMI adjustable rear control brackets I haven't installed yet either. Any ideas? I kinda think its pinion angle.

 

If the problem appeared right after the car was lowered, I'd look first at the pinion angle. If it's something that developed later, it could be any number of causes such as worn suspension parts, worn u-joint(s), and possibly broken/worn motor mount(s).

 

The car had a complete restoration and rebuild so its hard to say with just being lowered. I added an adjustable panhard bar Ujoints are new as well as the trans was freshened up. Im only suspecting pinion angle cause it was about the only thing I didnt account for. The rest of the suspension has poly bushings and all moog steering parts.