Ok I have an update for any have helped me thus far.
I have found out that it is infact a dead cylinder.
One of my cylinders are not firing and its cuasing the car to vibrate and run badly (obviously) )
I have went thru the basics of changing spark plugs, coil packs and wires but it still is not firing. So now I believe I have a very expensive repair, or the option to get rid of the car

 

Are you sure it's not firing? Have you done a compression check? It would still be an expensive repair, but I was just curious.

 

I'm curious if you ever got this problem resolved.

Because I have been experiencing something very similar to what you described for awhile now.
I have a '96 3800 with automatic trans
when I am in overdrive it will be at like 3000 rpm and then suddenly drop to about 2500. if i try to accelerate slightly when it is
at this lower RPM it will vibrate violently and make alot of noise.
if I accelerate hard enough to downshift, there is no vibration at all. This doesnt just occur in overdrive, if i put the shifter in third
it will behave similarly.
I just assumed that it was my transmission and that it would eventually just give out, but its been limping along and getting progressively worse for like 3 years now.

 

i have a 96 3800 and im pritty shure i have the same problem as you if u find out what it its reply back

 

I have a 95 V6 3.4 and it also vibrates going at about 60-70 MPH. Once I pass this speed, or am below it, it does not shake. Its not a violent shake, just enough to make my hand on the wheel shake. Also, when my RPM's are between 2,500 and 3,000, the car kinda stalls on me. If I press on the gas a little harder, then it passes those RPM's and accelarates normally. BTW, just yesterday my car started to shake and sounds like it wanted to turn off whenever I was at a complete stop. After checking it out for a while, white smoke started to come out of the exhaust. After doing some checking, I came to the conclusion that its my fuel injectors. I have new sparkplugs AC Delco which I purchased on January and new wires from Bosch. I also changed the oil this weekend and put Royal Purple straight 30 oil. My car seems to be fine on the liquids. Anyone know whats wrong. I forgot to mention that a little bit of liquid came out of the exhaust. It smells like watered down gas.

 

Does is idle rough also? I just tore my engine apart and when I got it back together I had the same problem. When the car went into Overdrive at around 55-60 mph and the rpm's dropped to about 15 it would start shaking, I also had a rough idle too. It turns out the rocker arms were too tight. I tightened them according to what the Chilton's book said but I guess the book is wrong so I loosened them up and started from scratch and tightened them a little less than the previous time and now the car runs fine.

 

I have a 93 3.4L and I have the same issue... I checked all the plugs, gaps, and changed the wires... NO FIX. It shakes when the car goes above 40ish... give it some gas and it stops or shift from Over Drive to Drive and the raises the RPM's and it stops it. No check engine light.

I can tell it has less power than my other 3.4 camaro and runs much rougher..

Was this issue ever resolved? Anyone got any ideas?

 

I have the same problem with my 93 too. Has a little "miss" when just sitting in park. Can start down the road fine, then it starts to shake and make noise. It can be pretty rough, then will smooth out and keep going until it happens again in a couple miles. This seems to be a common problem, has anyone found a solution?

 

I have a 94 i had the same probs but I Gave the car a complete tune up. and even changed the fuel filter and got new tires and balanced em. it still shook around 55 but was good over that. so a buddy had me Pull the Trans pan and change the filter and put fresh fluid with some of the Lucus stuff and now is drives perfect. just an idea for the trans issue. also check your intake gaskets. when they start to leak it swtarts to make it run a little rough. but like i said these are all my fixes and ideas. i could be wrong

 

have u had it scanned for trouble codes?