I've been running a custom made adjustable, poly panhard bar and rear LCA(lower control arms) for over 2 yrs now and love em. Here's my thread about em:
http://www.tahoeyukonforum.com/forum/showthread.php?t=26022
Adjustable panhard bar is a must IMO, for lowered rigs. But I kinda had to do the rear LCA because I was getting a popping/binding coming from RLCA, due to being low. But I have noticed an increase in stiffness and less sway and all around better handling/control. Someday I will get the upper arms so iI can adjust back the axle to center. But for now, the beefier adjustable poly bushing LCA have improved the rear end.
I also have larger sways f/r, poly sway mounts f/r, shortened poly sway links f/r, trailing arm relocaters, helper bags ect..
If your looking to stiffen up the rear, beefed up LCA's will be better then the stamped steel factory ones. But the uppers are solid from factory at least. Spohns UCA are definitely better then the stock ones, but I want them to be able to re center my axle, and not so much for stiffness (but obviously superior to stocks)