I dont have a book for the 91 but I do for the 92. In 92 the 2.8 was not offered but the fuel pump relay was located on the firewall close to the fender on the drivers side for the 4.3. Do you run it low on fuel often? This can take out the pump. After market pumps suck too. I learned that the hard way . Some parts are better bought from GM. The fuel pump is one of them because its a PITA to replace. Try going to the auto parts store and asking to see what the relay looks like. Then go find it on your truck. Go to the junk yard and get one. Its an item that will only cost you the dollar or so that they charge you to get in to look around. Put it in your pocket. I alway grab fuzes and relays on my trips to the bone yard. They dont care about those small electrical parts.