Lowering VY SS Ute

I am looking at lowering the rear of my 2003 Vy SS ute..I have fitted 18" SS rims with 235/40 tyres...but the rear sits a bit too high..I was just wanting to drop the back so the Ute sits a bit more level...With all the options of Sl,SSl,SSSl and Ultra low springs im not sure, which would be the best to go for? Do I need to replace the shocks as well or just the springs..Is this an easy job if just springs

Any advice would be great

Thanks


Alan