Re: diff swap or lockers
honestly the best benifit would be lockers front and rear... The transfercase you have is an open style, so in reality you only need 1 path to the ground, you put 1 tire on a patch of ice and 3 on dry pavement and you may be looking for a push. A locker in the rear would be helpful ish, that is unless one of the front tires has the least traction then it will again, just spin a front tire. IIRC there is not a locker made for the front of the grand which then goes to swapping axles and manually controlling power to the solenoids, which like the last option would be helpful but limited slip will still slip with enough torque differential.
Both options are spendy, the rear locker obviously being much cheaper than both axle assemblies, but less useful also... at the end of the day my opinion would be both are more expensive than the gains are worth. but obviously that is MY opinion. At least with the NV245 transfercase you could lock the transfercase so a rear locker would ALWAYS be helpful when needed.