A few years ago, I was shopping for fuel injectors and I recall being able to buy new ones for not that much more than remanufactured ones. I also ordered multiple "new" injectors off Ebay and they clearly were not new. Ended up returning them - twice. After that, I ended up isolating my issue (I was chasing a misfire) to figure out that the injectors weren't the problem so I never bothered replacing them.
Fuel injectors going bad is pretty rare with modern cars, particularly if you buy top tier gas. It's even less likely for those primarily using CARB gas (like what is sold in California and any other states that adhere to the same standard) - there are fewer impurities to clog up fuel filters and injectors.
The claims you heard are only valid if you already have tired/worn out injectors. Jeeps aren't sports cars so the benefit you'll see is likely minimal or just placebo effect. I'm always skeptical of the MPG improvement claims - yeah, new injectors can help if your current ones aren't functioning properly but, if you aren't having real problems (e.g. CEL, stalling, issues starting, etc.), the cost of the fuel injectors will take years to pay themselves off.
If you aren't having issues and are just trying to "tune up" your Jeep, being up to date on your scheduled maintenance is the first step. I'd start with spark plugs, running some fuel injector cleaner, and replacing your engine air filter - cheap and easy stuff. After that, I'd consider replacing your ignition coil (if you've never replaced it and you're up there in miles); testing your O2, crankshaft, camshaft, etc. sensors; fuel filter (not sure if these are replaceable on Grand Cherokees, TJs don't have a true filter); and cleaning out your throttle body and IAC sensor.
If you're really feeling like getting your hand dirty, service your transmission, transfer case, and front/rear differentials. Coolant system would be a good exercise too if you've never had it replaced.
