IF it is a 3/4ton, you are about as smooth as you are gonna get. If it is a 1ton, there were some shackles called Velvet Ride, I forget who makes them. Or you could go to an air ride suspension. Is it 2 or 4wd? 3/4 or 1ton? Big difference between a 2wd and 4x4, than add D/W350 springs in the rear.....
Guess you could put 2nd Gen springs on there, but it would involve changing everything, possibly even the axle. I know the springs are wider, so you'd have to get everything new to put on the Dana70 -u bolts, braces, spring hangers and shackles. Plus, I think the springs are longer, so you'd have to address ride height, driveline length, etc, etc.
Daniel