Re: vibrations--see if U can figure it out!!
The slack that you noticed is the big key. Were the u-joints new? I'm thinking they are since they most likely came with the cv drive shaft.....could they be damaged?....hard to say....you should have gotten a lot of miles out of them....
Have someone rock the jeep back and forth in gear while you are under it....if you hold you hand on the drive shaft, you should be able to feel the slack...and maybe even see it. I'm also wondering why the vibration got worse with time. Did the springs settle? Once again, check the angle, if its right then it u-joints are suspect. Any chance your getting excessive spring wrap?...I wouldn't think so on a non SOA.
Lets us know what you find....goog luck.