1989 GT 5 speed 3.73 gears

I thought it might have been a bent rim or bad tire so I swapped all the tires for ones that I know are good. I still have a vibration that starts very little at 50 mph in 4th gear and a little higher in 5th gear.. At 80 mph the seat , and shifter are violently shaking/vibrating.. I almost cant get it into 5th if I take 4th gear up there.

The only shop in town that would look at it is not open today.. so I am curious if I can diagnose it myself , short of throwing the car on stands and letting it run in gear.

Could it be the U joints ? old transmission brackets ? unbalanced drive shaft?

I don't plan on using this car to drive for more than a month, but if I can fix this I might end up using it as a daily.. Its fine if I don't go over 55 mph but I have to drive on I4 one day a week.. so it would be nice to get up to 70 and not have things vibrating in the car.

The car has had multiple accidents from what I can tell.. the rockers on either side have been buckled up and the car has obviously lived on a dirt road its whole life. The Trac-Loc is working fine, and axles have less than 1/8 inch play in and out.

The rear shocks and springs are good, front springs are good , but it has two different front shocks that suck.

I also clocked the drive shaft 180 degrees with no luck.