Side to Side Shake @ 30mph

Chrysler 1995 LHS

I bought this car two years ago, and since then I've had this problem in the front left where it rumbles a bit as I would go around corners with bumps slow. During the last week, it started getting much worse with bumps and pops here and there. Last night, I was just pulling the car into a hotel parking lot (weekend getaway gone bad) and my car stoped and I heard a loud noise. I immediately put it in park, and the noise persisted, so I shut the car off. After inspecting the front end, I realized my front left wheel was up against the wheel well. So this morning, I called and had it towed to Sears, as it was just down the street. Sears looked at it and said I had broken a ball joint. They replaced that, and at closing time tonight, sent me on my way. After the first block, I noticed that my car had issues... the whole car was violently shaking side to side when I get up to around 25 to 30mph. If I let off the accelerator, it immediately stops shaking. If I tap the accelerator again, it starts shaking again. I took it back to the shop, caught someone still there. He drove it around the lot. We discovered that it didn't shake when turning, only when going straight. After putting it back up on the jacks and shaking the tires around, he thought that it was the right ball joint. There is a minimal amount of play in that joint... I shook the tire and you could here it click. However, I'm convinced it isn't that ball joint for the following reasons: 1, it stops shaking immediately upon releasing the gas pedal, and 2, I've never had that shake before prior to bringing it to the shop.

I'm at a hotel tonight, and they're going to take another look in th emorning, but after already spending $400 on a 12 year old car, I'm trying to keep my costs down. Any ideas as to what the problem may be? Suggestions on not getting ripped off with this?

Other notes: The CV boots looked good to me, and the mechanic that was there. I saw no visible signs of leakage or damage. Could damage be there with no signs of it? The shop was also aware of my shaking problem before they gave the car back to me... they mentioned it and blamed a tire. The mechanic said that the tire looked like it might have a broken belt, but then he couldn't find evidence of it after it was off the car. My full size spare is now on the car with the same issue. He thinks the tire might have been fine all along now.

Thanks in advance.

I received a call back from Sears this morning. They said it was the drive shaft. The part comes in at 2pm today, hopefully that's all it is. If this sounds a bit odd, let me know. Thanks in advance.