Unusual vibration

first of all...where can u feel the vibration come from....floor or steering wheel? front or rear of car or just all over?
a while back i had a nasty vibration throughout the entire car, except at about 45-50 mph, and if i stomped on it past there, the vibration would be pretty minimal. my driveshaft was out of balance and 'unbalance-able' and it progressivly became worse at it tore up the tail housing bushing and then the output shaft bearing on my c4. my started as soon as i did a gear swap, i never noticed a vibration prior, even up to 100mph. but as soon as i swaped from the 2.78s to 3.55's the problem started.
driveshafts arent the cheapest so i dont kow if you want to just replace first, but if it is the problem, and you keep driving it, it may tear up ur trans like it did mine.
you say you checked the u-joints and they were ok, you may try replacing them first just in case, they are cheap.
you may also want to check the rear wheel bearing retainer bolts, maybe the are comming loose?
if its from the front/steering wheel... hit any curbs lately? alignment? tie rod ends bad?

the vibration was still there if i was faster than that 45mph, just not as bad when i stomped on it...there was no way i could drive on the freeway at a constant speed.
is it the original driveshaft? i had started a thread when i was having my problems and a few people had said the old driveshafts go bad and arnt worth fixing. so maybe that could be the reason it started all of a sudden and is getting worse...

You have to first visualize the frequency of the vibration. A drive shaft vibe is completely different from a tire vibration. The two are not similar at all. You say it's there on acceleration but not on decell, that suggests to me a driveshaft or U joint. Did you remove the driveshaft and check the joints, or just check for slack in the driveline with it in the car? Most U joints when they start to go bad, cannot be diagnosed by just checking for slack in the driveline. You have to to remove the shaft and work the joints around to feel the rough spots.

As above, symptoms sound like driveshaft. There is a rule-of-thumb about vib'ing under accel vs. decel as to what the problem is, but I don't recall it. It you have a Shop Manual, it has a troubleshooting guide that would help.

We are asuming that you have not had the DS off recently and put it back 180 out, true?

KillerKittyCat said:

No, just did an in car. My experience with u-joint failure in the past has been vibration and grinding. I do not have any grinding noise.

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When joints first go bad, they'll vibrate, you won't hear anything, only feel the vibes. Take the shaft out and work the joints around by hand, you'll feel the rough journals if there are any. Once you feel it, take the joint out, and you'll be able to see the damage to the journal surface, small tiny ridges. If you wait till you can hear them, you've waited too long, and will risk a dropped driveshaft.

D.Hearne said:

When joints first go bad, they'll vibrate, you won't hear anything, only feel the vibes. Take the shaft out and work the joints around by hand, you'll feel the rough journals if there are any. Once you feel it, take the joint out, and you'll be able to see the damage to the journal surface, small tiny ridges. If you wait till you can hear them, you've waited too long, and will risk a dropped driveshaft.

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I concur with this completely. I just replaced the ones in my 95 cobra and they visually did not have any problems. The vibration was progressively getting worse and as soon as I dropped the shaft, it was very apparent they were the problem.

Good luck