Rear shocks for truck on 4" lift.

I have a 2006 ec/lb Chevy 2500HD with a 4-6" Cognito lift set at 4" in the front with a 2" block in the rear. I'm running the factory leaf springs with firestone air bags. When the lift was installed about 2 years ago I got the Bilstein 5100 shocks for all 4 corners. As of right now one of the rear shocks is blown and the second one could be as well since I haven't looked in a while. I plan to swap the factory leaf springs out for 3500 drw springs with the extra over loads on top in the near future as well. Is it worth spending $70-80 per shock for Bilsteins again if they don't last or since the truck is/will be sprung pretty stiff in the rear or will some cheaper $30-50 Rancho's or Skyjackers or something work just as well? I don't want it to ride like absolute crap, but it's a work truck and I can live with a rougher ride but I don't want it to ride like it's blocked solid in the rear.

I would just go for the cheaper shocks since it's a work truck. Cheaper and it doesn't hurt as bad when you need to replace them. You could also get some high quality rebuild able shocks if your planning to have the truck for a long time but that can get pricey