Pacer Performance Flexy Flares extensions are an excellent idea if you need to add on some flare width to cover your treads for legal reasons or simply to keep the spray down. Work well with stock Jeep flares (lip mount) or without (surface mount).
These mount with the supplied hardware, and work well and honestly look pretty good. Mounted mine by myself in around 2 hours, but a second set of hands is really a good idea. Could have done a better job with 2nd person, but it is only small details that would have been improved. HIGHLY recommend the steel wire reinforced flares, they hold the complex curve of the XJ wheel wells better.
Bought mine from JC Whitney, good price for a set of 4. A good option if you don't want to drop big dollars for Bushwacker or Rusty's etc.
Rubber Fender Extensions Fender Flares - JCWhitney
Pacer High Performance
Rate them a solid 7 out of 10 overall for what I am using them for.