the way I see it is, you can call Rubicon express and see what shock they recomend for their 5.5" spring, get the extended and compressed length of the shock and find out how much droop and compression you will have from ride height, if you haven't already installed everything and want to get the shocks before you start.
The easiest way is to install the springs, mock up where the bar pin eliminators will leave your shock mounts, and find a shock that has a travel range that will work out for you.
So just for an example, say your shock mounts end up being 20" apart at ride height, measure how much compression you will be allowed, and the same for droop, say 6" up 6" down for simplicity, then you would need a shock with 12" of travel, with an extended length of 26" and a compressed length of 14", these are all numbers pulled from my head so don't go off of those. Chances are you will not find the "perfect" size shock and you will either have to add bumpstop or limit travel, compromise in some manner.
good luck