The ECM on the jimmy, is located on the passenger side fender well by the overflow tank. But before replace the ECM, inspect the fuel pump and fuel pump relay. The ECM sends power to the fuel pump relay and if the relay has gone bad will cause the fuse to blow which will kill the the power to the ECM, in turn kill the electrical power to the ignition system.
Start by removing the fuel pump relay and swap it with another relay and see if the fuse will blow again, if is does. Disconnect the fuel sending unit wire harness at the frame and see if the fuse blows again, if the fuse stays good. Chances are your fuel pump has gone bad or the fuel filter is dirty/clogged causing your fuel pump to draw more amp and will need to be replaced or the wire harness from the frame to the sending unit has a short which is common on older Jimmy's.
Here is a link to the electrical harness throughout the Jimmy.