The rock krawler joint uses a pretty short shank and I doubt there is enough threads to make the arms long enough to re-center with the addition of an adjustable upper set. This is pretty well know and have seen several threads about have to "upgrade" the lower arms when adding adjustable uppers to re-center the axle.

If you swapped that joint out to say a currie Johnny joint or the metal cloak rod end then you should have plenty of extra threads to extend the lenght of the link. Note that rock krawler uses a 1" shank on the rod ends vs 1.25" used by others makeing that a weak point.

 

Here are the SKU's.

RK02022 - Rear Lower Control Arms
* Requires Adjustable Rear Uppers

RK02064 - Rear Lower Control Arms
* For use with Factory Rear Uppers

Copied and pasted from RK's site.

High Clearance Rear Lower Control Arms

Take a look at the tab that says "How to order - SKU's"

 

So the mid arm system is not easily upgradable if you want add upper arms down the road. You would have to get rid of the rk lowers that came with the kit and buy new lowers and uppers. If that's the case then that really sucks.

 

Oh, come on, everyone loves a kit that is "upgradeable" it is what makes it special.

 

If you do the 2.5" Flex Kit then it is easily upgradeable. Its the Max Travel that would require you to discard the lower rear arms if you wanted to install the upper rear arms.

So this issue only pertains to the 2.5" Max Travel and the 3.5" Flex kit.

You would always be able to sell the lowers to recoupe some of the money.