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.

 

I think I am going to clock the drive shaft a time or two more and see what it does.. a little later that is.

 

Clocking the DS wont do anything. Inspect the driveshaft U-joints. Id pull it out and replace them.

 

ok, thanks. I'm going to do that next then. Is the trans going to drop its life blood on me when I pull the shaft out?? I have the car backed into my drive at the moment. . so the front is pointed down. .. towards my sinkhole

 

Shouldnt leak much at all like that. Put a lil pan under it anyway.

 

ok. Ill have the back end jacked up as well to work on it so..

I did have a bad experience with a 240 in my garage once , looked like a classic BP incident

 

You should throw it in neutral when you get up to speed to see if it still does it. You can then bring the rpms up to what they would be in gear to determine if it does it neutral, with a load in gear, or just coasting. That will definitely help in diagnosing the problem.

 

Any time the car is rolling at high speed.

speeding up in gear , coasting in neutral at high speed, I can barely get it from 4th to 5th at that speed because it is shaking / buzzing so bad.

 

Then I would definitely check the u joints. If it was me I would also check the wheel bearings (if coming from the front).

 

still vibrates like hell

did both u joints and new rear seal on the trans. the rear bearings and seals are fine..

I even clocked it back 180 degrees to where it was when I picked the car up.

I'm not going to put anymore time into it. It has some bad oil leaks and its really really pieced together.

I guess now Ill decide what parts to strip from it and junk the rest. I am not going to put more than a couple hundred dollars into it when I don't really need it.