need an engine it is a 5.3 what is the difference of the 2 5.3 engines available so I know the right one for my truck?
You need a Generation IV engine.
From Wiki. The Generation IV 5.3L engines share all the improvements and refinements found in other Generation IV engines. 8 versions of the Gen IV 5.3L engine were produced: 3 iron blocks (LY5, LMG, and LMF) and 5 aluminum blocks (LH6, LH8, LH9, LC9, and LS4). All versions featured Active Fuel Management except for the LH8, LH9 and LMF.
Look at the emissions sticker under the hood to know the exact version you have.