Bad vibration on 1989 Mustang GT
 

I noticed that I have a bad vibration on my car especially when I decelerate from approx from 80 to 65 mp/h and it feels like I was driving on the "rugged" band on the side of the road which is not the case...I don't feel this vibration on lower speed like 60 mp/h and less in lower gears though...I have new tires, freshly balanced and aligment just done...Any ideas of what could cause that problem??

Have you checked your driveshaft or harmonic balancer???
Where is the vibration??? Front or rear of the car???

I have not checked anything yet cause I noticed that problem this morning after my night shift...I will check the DS but I don't know what the harmonic balancer is...my mechanic must know...and the vibration seems to come from the rear of the car but I can feel it in my shifter and a little bit in the clutch... mostly in 5th gear...

Sounds like a DS issue. Do you have aftermarket gears in the rear??

Yeah,i would check the driveshaft,might have a bad u joint ready to go.

Nope...stock 2.73s...

I hope this is it...cheap to fix...

I would agree with everyone else. sounds like te u joints to me.

could be the front bearing in ur trans is worn

I would start in these steps...

Start with this step since its free!
Pull your SPOUT out and check your timing....if not tampered with it should be at stock 10* Adv.
If adjusted it will usually be in between 10*-14*Adv.
If timing is way off more then likely your Harmonic balancer is spun and needs to be replaced.
Replace balancer and run car. If all is good move onto the next step.

Replace your U-Joints since they are cheap and probly need to be done any ways!
Run your car....
If problem still persists you have a weight missing off your drive shaft, or it has been hit/damaged and means you need to upgrade to an Aluminium drive shaft witch should be done anyways due to weight reduction and less rotating mass LOL.

OR

There could be a bent rim, broken belts in the tires, or even a flywheel problem witch I doubt but ive seen worse things happen lol.

90% chances your U joints have had enough and is ready for retirement! and you can do one of 2 choices
1) Replace U joints
or
2) Buy an Aluminum drive shaft since it comes with new ends and U joints ready to rock and is a HP gain I guess you could say!

Good luck and keep us updated and what it was!

Thanks for your input guys!!! It was actually the U joint as many of you suggested and the problem is now fixed!!:tup

Thanks again!!