Vibration problem getting odd..
 

So after I figured that I need a new rear crosmember, I decided to put the car up and tighten everything.

After doing so, the vibration went nearly completly away. For about 4 miles.

These are the symptoms:
Harsh vibration 3-5th gears (40mph+).
Stops when releasing gas pedal.
Comes back quickly.

Im thinking I need to do what the last owner and shop failed to. Put locking washers on things.
The driveshaft bolts were about a 1/4turn loose all around. The tranny/engine bolts where loose about a 1/4 turn also.

Other then locking washers, new tranny crossmember, should I be looking into anything else?

Rev the engine to redline slowly in neutral. See if you get vibes. Mine did the same thing for a week and then the engine went.

Ill have to check..... but im sure the motors fine.... rebuilt just a while ago.

it could be driveshaft u joints going bad

Could be a tranny going bad, What does the tranny fluid look like? I'd suggest replacing it if its drark.
EDIT: COuld also be mounts, check the tranny and differential mounts. Could also be U-joints as stated above.

When I installed the driveshaft less then 3wks ago, the joints looked good.
The tranny is also a low milage pull just installed. The fluid needs to be replaced/toped off.

Im going to replace the driveshaft bolts when I get a chance.

You say you had work done on it? Then most likely, they busted the rear tranny mount sitting on the crossmember. Just happened to me, can't go over 40 without shaking like crazy. The damn mechanic tried to remove the mount assembly without jacking up the tranny, had to pry off the crossmember and resulted in breaking the mount:mad:

I did the work.......... and I just droped the whole crossmember.

But the rubber was SHOT around the bolt. But im pretty sure this will fix it.....
$130 down the drain......(crossmember)

WHY cant that damn rubber insert in the crossmemeber be sold seperatly????
Its all I need!! :(

$84 poorer and it will be here next wednesday.
Until then Im going to fix the cabin noise and retighten everything good.

One Bolt
 

I have a similar problem for about a month it came out to be a missing bolt that goes in the tube from the transmition to the Diferential , although all 4 where tighten as hell missing one seems to cause some type of in-balance, after tighten all of them problem was gone. Sorry if I don't know the name of the piece of tube that goes in between but you will figure out what I'm talking about in spanish is "Caldan"

Later....

You mean a drive shaft, yeah those are the bolts im replacing and using locking washers on.
Hope I havent lost one yet!!