Where's the ECU and how hard is it to change?

The computer in that year Suburban should be directly above the glovebox. Open the glovebox and peer through the gap between the glovebox and the top of the dash and the computer should be staring you in the face. To remove, I think you remove the 3 bolts from the bottom hinge area of the glovebox and remove the glovebox. Then, pull the couple of retaining bolts off the computer, and then remove the connectors to the computer. It should fall out then.