Get hockey pucks and slide them inside the spring. Save your money.
I didn't listen to this advice and bought these bump stops for the front http://www.quadratec.com/products/16311_43X_PG.htm
Got the 3" ones thinking they'd be good. Turns out the ZJ has 3" bump stops stock! So these do nothing if you replace the stock bump stops. I thought of leaving them floating with the stock bumpstops in place as well but they roll over sideways in the spring. I eventually replaced the stock front bumpstops that were shot with these and will be adding hockey pucks.
PS: I know you're supposed to drill the axle and bolt those bump stops to the bottom, leaving the stock ones at the top, but I don't like putting holes on my jeep, especially in the axle.
Worse case you can order the bump stops in my link above and use those on top in place of OEM, they are a direct fit. If you do that, use alot of WD40, those bolts holding the front bumpstop retainer cups are a PITA to take out. One of my bolts broke and it took me 6 hours to drill it out!
Then use hockey pucks to get the added bump stop length.
I have my bumpstops extended using 3 hockey pucks per side. I think I still need one more per side up front to stop my shocks from being the bumpstops. For the rear RE makes a kit. That's what I'm running and that's the only thing I've been able to find. I believe it extends them long enough to prevent my shocks from bottoming out.
Complete set of bolt-in extensions. Works perfectly. One set of pucks is used up front, and two each for each rear bumpstop. These space out the buckets from the unibody, so your OEM stops go back into the OEM buckets. IIRC, the hardware was actually allen-head bolts.