yes they are legal in JeepSpeed 3 and there will be atleast one running, I know cuz my buddy is running one of his. There still are some limitations (15" trav front, unlimited rear, and motor that was in a jeep, etc) go check out www.jeepspeed.com for more info on it. Grand Cherokees can run Wranglers,wrangler unlimiteds, pretty much anything Jeep.
It's about time the MJs got respect! My buddy is actually one of the main reasons Clive Skilton (owner of JeepSpeed) let the MJs in. I am stoked, he is racing a Long Bed w/ a 4.7L V8. He isn't sure if he is gonna run 4x4 or 4x2 yet, but he's paid his registration and got his number. :brows: