So I replaced all the bolts and the it helped the vibration a lot! It's drivable again, but the vibration is still there, I didn't check the front flex disc, which I want to check out tomorrow, my question is though, how do you take out the rear flex disc? When I unscrewed all the bolts it didn't come out, I mean I could pull on it but it still didn't come out, was I supposed to disconnect something else?

My dad keeps suggesting it's in fact the transmission mounts, so I'll check those out tomorrow too, that's just a visual inspection right? Are they easy to spot or do I have to dig under stuff to get a clear view. I really want to figure this vibration out, I'm getting paranoid it could be a flex disc or something LOL

I can't get it to the mechanic right away due to his backed up schedule so I'm going to try and see if I can diagnose this myself, I just thought for sure that replacing the all the correct size bolts would have fixed the problem.

Refer to my Post #4 for information about how to remove the rear flex disc.

So I finally found out the culprit to my vibration! It wasn't the flex discs which are actually in ok shape, it's my driveshaft center support...The mechanic looking at it said it was "gone" and he was able to freely move my driveshaft up and down with his hand. He said it won't hurt it continuing to drive on it like that since I've been driving with it like that since we bought the car, but I can't wait to replace it!!!!!!!!

Finally a ride with no thuds and constant vibration, I've never felt a "smooth" ride in this w124 and I'm so eager to get that!!! :D :D

The reason the center support was shot is most likely due to the cheezy flex disc connection. I would still replace that rear disc with a new one and new nuts & bolts. You also need to check the mounting holes on both the differential and driveshaft yokes to be sure they haven't worn out-of-round. An inspection of the front disc is also a good idea. Whoever was wrenching on your driveline last didn't know what the hell he was doing.