I wouldn't recomend welding a D35 for Moab, at least if you do I hope you don't run a trail that I'm on. The traction there is very good, and like Handlebars said, it's a PIA if you don't have selectable lockers (off) just driving on most of the trails. I run 2wd low almost all of the time there to keep the drivetrain from binding up and so I can steer.
If you're going to replace the D35 soon, put the locker in the front, it will give you traction if you lift a tire. If you're going to run the D35 for awhile, then go ahead and put a locker in it. A rear locked rig can do amazing things in Moab. I'm not much on welded D35's, they are just an accident waiting to happen. I certainly hope that everyone who has one in Moab is carrying spare axles, and knows how to replace them.