If you've got ESOF, then you can interrupt power to the hub solenoid. Shift into 4low like normal which puts the transfer case into low range, but drop power to the vacuum solenoid for the hubs.
I haven't done it yet, but was thinking about using an upfitter to accomplish it. I figure a normally closed relay (so in normal position -i.e., power off to the relay, the vacuum solenoid operates normally) then when you flip on the upfitter switch, it sends 12v to the relay causing the contacts to open, dropping power to the vacuum solenoid and leaving the hubs unlocked.
The other way to do it would be to break power for the vacuum solenoid through an aftermarket switch mounted somewhere on the dash. When the switch is closed, hubs operate normally; when the switch is open, hubs unlock.