RogerDodger1

To diagnose the vibration problem, first of all we need to know where the vibration is being felt. Is it in the steering wheel, or the seat of your pants, or both? If it's in the steering wheel, it's most probably the front tires, but if it's in the seat of your pants, it's most likely the rear tires, (both probably an out of balance issue). One of the things you can do to diagnose the problem is to drive the car at the speed it starts to vibrate and without any traffic around you, put the car into Neutral and coast. If the vibration goes away, then it's a driveline/transmission issue. If the vibration persists, it's not a driveline issue, but instead an out of balance tire, a bent wheel, an out of round tire, or all three.

As for the whining from the transmission, this could be a lubricant related failure. Many motorists are religious about changing the oil in the engine, but neglect the transmission and rear axle. Go figure. It's possible that changing the fluid might help, if it has been neglected and never changed, (which is common). It is also possible the incorrect fluid was installed in the transmission, as many people assume that all standard transmission vehicles use gear lube in the transmission and this isn't so.

A 1990 Mustang with the 5-speed transmission is supposed to use 5.5. pints, (or almost 3 quarts), of automatic transmission fluid and not gear lube. See below for the Ford recommended fluid, capacities and torque specs for the drain/fill plugs -

http://www.amsoil.com/mygarage/vehic...FORD+MUSTANG+E

It is most likely though that the transmission has been damaged and may need replacing. It wont be cheap, unless you find a used one and that can be had at the link below which is a nationwide data base of junkyards all across the country where you can find virtually any part you might be needing. But even a used transmission is going to be a good chunk of change -

http://www.Car-Part.Com

Is this car stock, or has someone modified it? If it's been modified this can greatly complicate the correct diagnosis of the problem as who knows if the person working on the car was a qualified automotive technician or merely a shade-tree mechanic.

I hope this has been of help. Keep us posted.

Danny_Boy

This is so true....

The first things i did with my Mustang was FULL fluid and filter change ASAP,this included the rear diff.Then it was cleaning everything i could.

Man you guys should have seen the K&N cone filter,it was friggin black!! Not to mention it was a HAI (Hot air intake) system

19stang88

I had a similar problem. I thought the whine was coming from the gears. But when i changed my u joints, the whine went away, along with the vibration.

SnakeConvert98

is it four spd or five? someone could have put a jerico 4 spd in it and they wine by design!