If stock, your '91 should have throttle body injection (TBI) not a carburetor.
You don't mention seeing any trouble lights on the dash, so I'll skip the talk about issues with emission components, but report back if there are any.
First, check that air filter. This is a no-brainer...no air, no go.
Next, check your throttle body and vacuum lines for leaks by spraying short directed shots of carb cleaning fluid around the TB gaskets, vacuum lines and their parts while the engine is idling. The engine should stumble when you hit a leak. TB gasket leaks and cracked vacuum hoses are common on older trucks.
Next I'd pull your spark plugs and "read" them. The deposits and color on the electrodes can tell you how each cylinder is running. There are many color photos online showing what each plug condition means, but the link below goes a bit further in explaining how to service them:
http://www.aa1car.com/library/sprkplg2.htm
For you, you're looking for sooty deposits, which is an indication of too-rich combustion...too much fuel for the air to react with. If you see the soot on all plugs for both cylinder banks, you may have a faulty fuel regulator or worn injector seals. Seeing the deposits on just one bank could indicate a clogged injector (those two, fat, bullet-looking pieces just above the throttle plates). Each injector feeds fuel to the respective cylinder bank.
Next, check your injectors. A trick if you have an inductive timing light: hook up your timing light and attach the pulse clamp to the #1 spark plug wire (forwardmost, driver's side bank). Idle the engine and point the strobe at the driver's side injector. You should be able to see the fuel spray pattern in stop motion. It should be a nice, conical spray with no big fuel drops falling into the throttle body. Look at it from different angles. Do the same for the passenger side using the #2 spark plug (forwardmost, passenger side). Any irregularity in pattern or pulse length means they're clogged and need to be cleaned. You can examine the spray pattern without a timing light, but it's much harder to see the details because it goes so fast.
Finally, check your positive crankcase ventilation (PCV) and exhaust gas recirculation (EGR) valves. Make sure they are operating smoothly and not gunked up or clogged.
Good luck!