Spacers?

I've noticed on a few threads people talking about having 2" spacers on their XJs. What is this? Is it like the XJ version of the wrangler body lifts?
Are they worth having? If so where can I find them? Sorry about all the questions.

2 inch spacers would normally refer to coil spring spacers in the front. They are not like TJ body lifts, as the XJ has no frame. Only way to lift an XJ is through a suspension lift, hence the spacers in the coil springs.
You can get spacers of varying sizes from DayStar, or from any number of vendors, look for coil spring spacers.
Keep in mind if you lift the front, you will want to lift the back, and then the issues start, like Brake lines, bump stops, drive shafts, pinion angles, driving characteristics and the list goes on.

Notice I didn't answer the part about are they worth having?

Gotcha.
I have a 3.5" RE super flex installed. So maybe I dont need it then.

Well here is an example of where you might use them.
When I installed my first 4.5 inch lift, I added an ARB Bull Bar front bumper and a Warn M8000 winch. All this weight took the front end down, so I added a 1 inch spacer (actually 1 3/4 inch total, stock isoluator is 3/4 inch) to lift the front back up so the XJ sat sort of level.