You said the Rubicon you want is ready to go, with the mods you already want? Like 35's and all the assorted "stuff" to go with it? 19K seems spendy for a 15 year old vehicle, but really, it might not be bad. Good parts are expensive, and 35's are no small feat on a TJ. Takes a lot of expensive stuff to get there. I would want to know how it got to be running the 35's though. Did they just lift it and slap them in, or does it have the "right stuff?"
BUT, if you really wanna go after 35's on your X with a new set of axles, you'll have that money into anyway...and maybe more. You'll have stronger lockers, but a less desirable T-case. The AC is something to consider too. Factory air is always better integrated than aftermarket.
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@JEEPCJTJ said. If you are going to stick with 35's, Its hard to beat the value of the Rubicon. You wanna go bigger? Probably better to strip and rebuild the X. Honestly, if it were me, and I were trying to build a monster TJ, I'd even sell the X. Then, I'd pick up the cheapest, low mileage, rust free salvage I could find with a 4.0. Strip it down and put new axles, suspension, T-case, etc into it. You're basically only buying the engine, transmission, frame, and tub at that point. Even if the tub is dented or it has crushed fenders...At that level of build, you're going to install armor and probably different fenders anyway.