When I bought my '03, the SVT mech at the dealership said that if I was going to lower it using shackles to put them on the front of the rear springs. You'll maintain better angles on both u-joints and won't need shims.
Check it out and good luck.

 

I tried the 3's and they truck didn't handle correctly, I didn't use shims and could see some torsion bend on my springs. I went to the 2's and life was good, handling was back to stock with much less pressure on the "leafs".

 

Well i just picked up the gm shackles from o'reillys. It says its a 2 inch but i guess i'll find out this weekend

 

I THANK ALL FORE YOURE INPUT AND REALLY APPRIECHIAT IT,(I CANT SPELL! LOL.)THANKS AGAIN

 

I have the 2" shackle on my 03. (SVT-KY, gave them to me ) I too noticed around 20-30mph I have a vibration that I did not notice before. I was told to get cam bolts, and that should fix it. Is this right??? Any help on this??

 

Cam bolts (camber) are for the front. The vibration is related to the pinion angle being off. Take it to an alingment shop and have them reverse the shims and do an alingment, which needs to be done anyhow. Do a search under my name and you can see what I went through and how to learn from my mistakes. Good luck.