I vote for the rear

The a front will put increased stress on the weakest parts of your drivetrain (the 7.5" diff, stub axles, and cv axles). In the rear end it will do a lot of good still and you are less likely to break.

The reason they say the front will result in better performance is because IFS inherantly flexes less than the rear leaf sprung axle. This means the IFS is more likely to have a tire off the ground and all the power gets sent to that free spinning wheel. Yes it will benefit more from the locker, because the tire on the ground is aloud to drive the vehicle. But unless it has the help of the rear wheels, trying to drag the truck along with one planted tire is very likely to break something.

Yes both are best, but if you are only getting one, get the rear one.