Todd,
It has been a while since I've been to the dunes, mostly because they started charging admission, but I am quite familiar with both Buttercup and Gordon's well and points north including Patton Valley and Glamis. I've been all over the areas both recreationally (1970 thru 1983) and on the job (1983 thru 1990 and 1997 thru 2004).
There are a number of other trails in the area including Picacho Peak (along the Colorado River) and the Kofa Wildlife Refuge.
I moved back to the Yuma area after I retired and am just getting started re-learning some of the off-roading areas. Last winter I went along with one of my neighbors, a winter visitor, and his off-road club, the local chapter of the RoadRunners. The trail they took was probably a bit beyond what many KJ's would be capable of. Every vehicle was running front and rear lockers of some sort, mostly ARB, as well as lifts and many with sliders. My buddy's lifted Rubicon fit right in. Their club does go on less challenging runs and I'm trying to get my KJ prepared so I can tag along and learn more of the area.
Another trail to consider, while not in Arizona, would be the Bradshaw Trail running from Highway 78 just south of Blythe, CA to Thermal, CA. It more or less paralels I-10, to the South skirting the northern boundary of the Chocolate Mountain Bombing Range, and is not that far a drive from the Phoenix area. I worked that area from 1997 thru 2004, mostly at night, and plan to return, many times, as a tourist. A stock KJ would handle the Trail, itself, but there are a lot of side trails that are considerably more challenging.
As I discover more areas I'll be sure to let y'all know about them.
Paul
P.S. I dug this up on the Bradshaw Trail.
http://www.off-roadweb.com/adventures/0 ... jeep_trail
Paul