All of this.

First thing to determine is what kind of vibration. It's not a rough track with a tight shock issue? Tight shock in bump on a rough track can be sketchy.

What tire on what width wheel? Soft sidewall? Narrow wheel for the tire? Radial? Some grow more than others, depending on tire construction and the wheel it's on.

A driveshaft is a high frequency buzz. When it's bad, it makes things blur. Think a die grinder with a bent burr. A tire is a much lower frequency. Think a tire out of balance or a brake pulsation at interstate speeds.

The shaft may be balanced, but does it have run out? Many places are unable to spin a shaft at the speeds we need them to be right. I have had a slight out of true shaft balanced multiple times. It was OK for a few runs and the buzz came back. Ended up in the scrap.

Tires have to be balanced. And I made it a habit to rebalance them about half way through their expected life, because they do go out.

4 link on your car is along for the ride at that speed unless you are making Top Sportsman type steam. Moving the bars is useless when looking at a vibration.

Cranking up the shocks is not the right way to adjust ride height. Adjustable mounts are. IF raising the car and/or eliminating the tires hitting fixes the problem. then I would want the sheet metal fixed so you can run the right tires on it at the desired ride height..

Vibration frequency. High speed is something in front of the ring gear. Lower speed, tire, axle.

Driveshaft, balance and run-out. Tire balance, excessive weight needed means the tire is faulty. Tire hitting the body somewhere. I'm skeptical. I've seen tires hitting bad enough that they would smoke in the lights without vibrating.

Easy stuff first.