What sz brake shoes do I need

From what I can tell on the rockauto.com website, there are two different shoes available for your truck. Both are 12" in diameter, but the difference is in the width of the shoes; standard duty shoes are 2-1/2" wide and heavy duty are 3" wide.

Best bet is to take a shoe in with you and compare it to what's available in an auto parts store, but if you want to order online you'll need to pull the drum and physically measure the shoes yourself to find out which ones you need. I'd double-check the diameter of the drum and measure the width of the shoes; that should tell you everything you need to know to get the right ones.