zw7 is better known as 'smooth ride'
the manual self level RPO is g65
no matter as they are always together.
Yes, you will need new springs if you go to a standard shock out back.
P/Ns for the original stuff might help you find replacements. I know KYB has a nivomat replacement rear shock (SR2003), though I have not yet been able to find a price on it.
(edit: jsut did... around $300 per side -
http://buykyb.com/catalog/product_info.php?products_id=2477 )
front - 12477738, CHEVROLET(X88), G M C(Z88), PREMIUM SMOOTH RIDE CHAS PKG(ZW7) ABSORBER KIT, FRT SHK (CODE T3Y) (ACDelco #540-182) - $183.62ea
rear - 12477516 SELF-ADJ MANUAL LEVEL CONTROL(G65), PREMIUM SMOOTH RIDE CHAS PKG(ZW7) ABSORBER KIT, RR SHK (CODE TW3) - $586.29ea