The Grand Cherokee (WJ) has a 5 on 5 lug pattern. I personally don't have any experience with wheel spacers yet, but when I get them soon, I'll go with Spydertrax. They are kind of "THE" high end wheel spacer brand and although there are much cheaper alternatives that are probably very good, Spydertrax are the only one's that I haven't heard of anybody having issues with. The main concern is to make sure you get some that are hub-centric. You DEFINITELY want hub-centric spacers. Any 5 on 5 spacer should fit your WJ, especially since they're mostly made for Jeeps (at least in that size) and the Jeep JK Wrangler is pretty much the main vehicle they make that size for. The JK and the WJ are both 5 on 5 and have the same fitment as far as the hub. So if you get any quality spacers that are hub-centric, they should fit absolutely.
Other cheaper brands that are hubcentric are Kevin's Off Road and Rough Country. I think Rusty's makes them too. They are all probably just as good as the Spydertrax, aside from maybe Spydertrax having somewhat higher quality aluminum or slightly better machining tolerance or maybe better lugs (just speculating here), but Spydertrax is the only brand I haven't heard of any issues with yet. Regardless of brand, you need to re-torque them REGULARLY. At least every time you rotate tires and I would re-check them consistently for the first few months until you're sure they're staying strong. Most people recommend Loc-Tite, which I would definitely do. I think most people who keep an eye on them and torque them regularly as well as Loc-Tite them don't have issues with the less expensive (but still quality) brands like KOR and Rough Country. But Spydertrax just seems like a better bet to me. I don't know if they invented that style of spacer or if they just popularized it and perfected it, but they are the main brand that introduced off-road worthy high end spacers, as far as I know. They're 100 bucks a pair, but I'll probably get them just to feel like I have the best/safest ones I could get. I would still be pretty confident with anything Kevin's Off Road sells, though, and I'm sure they'd be fine if you take care of them.
But yeah, you need 5 on 5 hub-centric spacers.