Engine vibration

I've had my mom's car for a couple weeks now and I noticed earlier that it would intermitently have vibration problems and the car would shudder when taking off a little. I've been having worse problems since then. She's already taken it into a mechanic, but because it was intermittent at the time he said he couldn't diagnose it. The car will really start to shake if I'm going about 50 and the transmission shifts up and the tach drops to in between 1700 and 2000. I've checked the spark plug wires, they're all connected, the oil and trans fluid are both good as well. I don't know what it is or why it's happening. At other speeds it still shudders but it's much less noticeable. Any ideas?

Also, more information, hopefully this helps me get some answers before 5pm. The LeSabre Limited is a 1994. I believe it's around 150,000 Miles. It pretty much seems fine when the car isn't in gear. If it's in neutral or park there may be a little bit of a shake when you first accelerate, but nowhere near the shake that I get when I'm driving. I also tried adding fuel injector cleaner into the gas and Lucas transmission stuff to the tranny fluid. Neither one has made too much difference. If you think it's the TCC please give me an indication as to where exactly I need to look for the plug (from the top, bottom, or which side?). Also if it helps, the car only does it if I'm pressing on the accelerator. If I let off of it it's fine, until I stop and if I stop while it's in drive you can tell that there's something wrong because you can feel the shake.

A couple of quick thoughts -

So it does this even when you're stopped and idling?

My '91 LeSabre had its coil pack partially go bad, which caused the engine to 'miss'. To check, turn engine off, mark where each plug wire goes and remove all six wires from the coil pack, and measure the resistance across each pair of terminals (there should be three coils). The resistances should be somewhat consistent. If one is double that of the other two, or shows an 'open' or short, then there is a problem. From my (one) experience with this, the coils appeared (again, just a guess) to be double-wound, so when one of the windings went out, the total resistance doubled, and the coil occasionally caused trouble (but not constantly).

There are lots of possibilities for this, but at least on the '91 the coil pack was a known problem area.... not sure about '94 though....

I just went and checked it, my 3-6 coil gets a reading of damn near open or open. While the other two coils seem to get about 006 or lower (I had the ohmmeter @ 2000k). So was it running on 4 cylinders then?

Seems like it :)

You notice it more once the tranny shifts into 4th w/ TCC engaged because of the low RPM and direct link between engine and transmission....

Hope it solves the issue for you!