I was told by both Rough Country and Jeep (DC) the springs in all TJ's (ie; 2003 - 2006) are the EXACT SAME SPRING. Check the part numbers for a 2004 Rubi and 2004 X. Their the same. Where the difference is is in the axles. The D44's front & rear on a Rubi sit 1/2" higher than axles on a Sport or X model.
For example, if you were to buy a 2.5" lift from Rough Country, on the Sport and X models, you will achieve 2.5" of lift, where as on the Rubi, you only get 2.0" lift due to the axles - not the springs.
Oh and as for the OP's question -- You can't appreciate the "specs", until you've wheeled a Rubi off road!
2004 'Quick Silver' Rubicon