Have a 2WD SC with 3 or 4" body and a 6" suspension lift. has a 75-80MPH vib. was tolt that I do not need a 2 peice driveshaft but could this cause it. the trans seems ok (pink fluid shifting no leaks) had tires and alignment ck also

 

If you have a 2 piece shaft then try adding and removing shims between the carrier bearing and the crossmember.