If you're buying them from Cognito, just get the part number they specify for your lift height (or no lift).
FWIW, i'd save the money and go with standard shocks instead of resi's. These trucks tuck the shocks up so far that you can't see them for the bling factor, and I doubt you're taking a 2500 high speed off roading so the benefits of the resi are nill.
With that said... im rebuilding some FOX resi shocks that I had on my Jeep for my Yukon 2500, but mine have the C/D adjuster on them so I can adjust the damping for daily driving/towing/beach driving. I was going to sell them and just get some Rancho 9000's which would be easier to adjust with the knob on the lower section of the shock... but by the time I sell them and buy the Ranchos, i'd probably only save 100-200 bucks.