Didnt some of the 3/4 tons come with 2.5" springs. I though I have ran in to them before. I could be wrong though. I do know when I have bought springs for the 1/2 ton there was the option for the 3" wide ones.
i wouldnt be worried about the rear axle. They have thoes in 3/4 ton trucks behind the deisle.
I might have 3/4 springs laying around.. Ill have to dig though my parts truck if your intrested. but I agree with the guys, you just need a bigger truck.