Quoting from Greg at PRG regarding the Radflo's:
"Different spring rates are used on the 2wd or 4wd versions."
2.5" for 4WD, 3.5" for 2WD. I am not sure how exactly to change spring rates (unless you have to swap springs), but that was one of the reasons for including whether it was initially for a 4WD or 2WD.
Edit: I retract this previous post...that is for 2.0's, and I just noticed seller has the 2.5's. My mistake.