I'm not totally familiar with the Hummer chassis (CC-01 I think is the number designation?) but if it does have a ball diff up front and gear diff in the rear, I would certain swap them if it's possible. Before I did any mods to the diffs, I would try that first and make sure they're interchangeable.

If so, I would go ahead and lock the gear diff because those are easy to lock; just stuff JB weld into the spider gears. Blue tack or silly putty should work for a non-permanent locker. I would leave the ball diff open. Ball diffs are hard to lock and certainly can't be locked in a temporary fashion. A rear locker should be good enough for a trail runner and an open front diff will keep your steering radius reasonable.

You don't HAVE to run-in the motor, but it certainly doesn't hurt. An old-school method is to hook the motor to a 4-cell battery (4.8V) and dunk it in water. The water washes away all the particles as the brushes wear in. You could also just hook it up to the speedo and run it at half throttle for a few minutes and spray the motor with motor spray to displace any brush particles.