As far as 'factory' rear bars that are an improvement over stock, the 24mm 2dr 98-05 rear blazer bar is a good option. I have the blazer rear bar and 34mm front bar with 5660 front springs and the car corners flat and only slightly understeers at the autocross, but I am attributing that to more lack of improved front suspension geometry. My car is set up for for drag use however so I'd rather have a stiff rear bar than soft.
Hellwig sells a sub $200 option for the same style axle mounted bar specific for G body use and belltech sells a larger diameter 28mm 2dr blazer upgrade bar for the rear for $260 that has new bushings and axle mount brackets which you are going to need anyways unless your salvage yard is really kind and uses a wrench to remove the U bolts instead of a torch.
What is the largest OEM rear bar that was installed on a G-Body?
gbodyforum.com