I think to put in a locker I would have to upgrade to a 14 bolt rear end.One vendor
http://www.eastcoastgearsupply.com/ stated that you can't put an aftermarket locker in a 12 bolt Dana 60. In which case the full axle would be a start.
I've had such bad experiences in the past with dealerships that I hesitate to deal with GMC.
If I'm dropping out the rear to re spring that's the time to swap the axle.
Fleet Authority would do it locally but the owner didn't seem like he really wanted to do it.
My gearhead nephew in NJ has the means but not necessarily the time. If I'm going to travel to get this done that would be the ticket.
I've a lot more digging to do.
Thanks for the replies.