Just adjust the pinion angle to make sure you aren't stressing anything. I'm lowered 2" in the rear but i've driven 110+ for long periods of time and never felt any vibrations at any point.
Long distances being 15-20+ miles at a time in some cases...... Don't ask.

 

nice 15-20 miles- when i go over 120 i get a vibration. i might have to try a custom 1 peice just to test it out. ill be the test subject and just keep my two piece just in case

 

before you get a 1 piece check your pinion angle and your carrier bearing...after i lowered my truck i got a vibe at 100 and it was my carrier bearing....imo i wouldnt run a 1 piece after seeing countless threads on breaking them

 

did you replace the carrier bearing?

 

yea i replaced it with a GM one....it was a revised design

 

I've taken mine up to 134 (GPS confirmed) with no vibration so I agree that you should check the angle. (this was before my 2" drop).

I don't know when the revised the carrier but Black Rado is 99-02 model and you and I are 03+.
so might check on when they did that.

 

Another thought..... What kind of tires are you sporting?

 

stock 2 piece with 4l80 yoke. Had all u-joints and carrier bearings checked, then speed balanced when I swapped trans. No vibes or probs

 

my drive shaft has never been touched. i dont plan on it either until i start feeling some slop in the joints.

 

what drop does your truck have