Car goes shaking when goes 65mph

I having problem with my car. My Mustang is 1988, 302, 5.0, shift, lowered, 211,000 km. When I driving and goes to 100 km/h zone then my car goes shaking and I thought its might tires but I realized where I holding my shift trannmisson were also shaking too. I going try install new tires since my tires were bald. And Some mech said it's might be differential. I talked to other mech and Said Differentail are ok but they never checked inside. It's could be shaft? I having hard time passing the car when somebody drive 10km/h slower. I barely make it but I could goes littler faster but its getting worth. But under 100 km/h is nice and smooth even when I drove on 5th gears. Anything suggesting? Thank!