The trails on URE are most certainly doable in stock KJ's *except* Kodiak rock. Even there (Kodiak) you have lines that can be done safely in lifted KJ's --- just don't take the one that Mutt did (he broke his Al's upper A-arm on the more aggressive line).
There are issues with wheeling the KJ. Mostly you just have to keep the bouncing in front to minimum if you can, because that aluminum front diff tends to grenade from gaining sudden traction. I was on a trail where San Diego Dave blew his front diff on a nondescript hill precisely because of this.
So that leaves Buckaroo Banzai'ing through sand dunes out, but rocks still in
Here are some pictures from Holcomb creek in Big Bear. Holcomb is rated just slightly below the "John Bull" (most difficult) trail in Big Bear, and the KJ does fine.
Rock garden at the entrance of Holcomb Creek
Second rock garden
This is the view from the middle of the first rock garden
But then every now and then you relax, whatever, and this happens:
the same unassuming rock claims another victim
That was the same trail (Cleghorn) that claimed my bumper for a long time (cracked it coming down on a rock).