Different engine size is just telling you what the axle came out of. There is no difference in the axle by engine size, they are the same, from the 3.9, to the 5.9.
You need the complete axle assembly. (minus the springs, unless yours are shot?)
Locking implies it has a limited slip differential. It's a traction aid. In a standard differential, the tire with the least traction gets ALL the power. With a locking diff, if one wheel starts slipping, the diff will transfer power to the other wheel.