[4" Rear Drop On A 05 Ss]

Hey guys I have had a hell of a time finding out how to get lower in the rear -

 

I already have the 2/2 drop from belltech on my 05 but want another 2" drop in the rear -

 

I've been told by some shops that this isn't even possible due to the factory drop and the shackles I have on -

 

Any help would be greatly appreciated