My rear end/drive line seems to vibrate in the seat at about 50 mph. It has always been this way. I have been reading here that it probably has something to do with either an unbalanced drive shaft, a U-joint that has a bad angle, or an axle being bent or warped. Perhaps even the tires are out of balance.

I was wondering if anyone knew of a reputable shop in Miami, FL or even up in Broward that could help me with this problem - I want someone who knows what they are doing with the 1st gens. Suggestions?

Not in Miami, but Denneys driveline in NY is top notch, you can talk to the guys building them and they do an awesome job.
I ordered mine from them and I live on the opposite coast, but well worth the wait, check 'em out.