I have the 4.5 superflex with sub'd 3.5 springs. I am still running the stock drive shaft. The working angle of my DS is 7 degrees that is 4 degrees above what DC says should be max. No vibes except on initial roll out, then smooth. If you have the cash go with a new DS especially if you do a new T-case skid.
I have the RE 3.5 and had a small amount of vibration without the transfer case drop. With the drop installed there were no vibrations at all. I have since lifted the transfer case even beyond stock with a Medic skid and installed a Tom Woods DS. No vibrations at all.
Eliminating vibration is obviously a very important part of getting a new Driveshaft but dont forget that the length is also very important.
When flexing, you want to make sure your driveshaft can slide far enough without coming apart.
How in the heck did you all keep your xfer case shift linkage together. After my RE 3.5 install and 33" tires the tub mounted xfer case bracket looked like it would pop the linkage pretty easy. I was nervous about that...and it had a bit of a rattle. So when I did the lift I also did the skyjacker xfer case relo bracket.