Since its basically the same as a s10 blazer id figure id ask here. It has the 4.3 vortec and we have replaced the battery, plugs, wires, distributer cap, rotor button. ignition coil. and no vacum leaks.... When in idle it idles great,as soon as u put it in gear it misfires on 3 and 5.,its drivable for a few min then dies. Could it be the fuel regulator?

 

The poppet valves on the central port injection (CPI) can become clogged or stick, preventing fuel from reaching each cylinder. Have you run any fuel injection cleaners (Sea Foam, Gum Out?) through the system?
The spark plugs may show if the affected cylinders are running lean. Some deposits will normally accumulate on a plug, so if it is too clean, the cylinder is running lean.