The rear drums usually get replaced much less often than the front discs, and considering that drum brakes are much harder to work on than disc brakes, you might as well replace them when you replace the shoes.
You may also want to replace the wheel cylinders, too. Rock Auto carries Advics wheel cylinders, made in Japan
Get the Wagner E-coated drums from Rock Auto. They are the best drums to get because their coating covers everything except the surface where the shoes contact. Most other coated drums don't coat where the drum contacts the hub and the wheel, leaving them prone to rust and impossible to remove.