Started by dying whenI got below 1/4 tank, then just died at a stop light a few weeks ago. Replaced the cap, plugs, wires, rotor- started fine after that. Just tonight died in almost the same spot after bogging down for a bit, still had 3/8 of a tank. Added more fuel and was going to try the fuel pump relay but can't find in the dark - no luck with the extra fuel. All it will do is try to turn over.Any ideas on what I should check and try?

 

check your fuel pressure it should be around 60 psi.that is if it has the vortex motor in it.change your fuel filter. the relay is on the driver side along the fire wall

 

they are all vortec engines the difference is if it is a TBI or CFI injection system. CFI (vin code w) will have a flat plastic piece on the very top of the engine that says vortec (unless somebody removed it). TBI will look similar to a carbureator (vin code z).
You should not be driving these trucks with under 1/4 tank unless you like replacing fuel pumps (they tend to overheat and die quicker than normal when run with less than 1/4 tank of fuel)
If your standing at the front of the truck looking at the master cylinder/power booster for your brakes the fuel pump relay will be just to the right and slightly behind the power booster (see pic below the clean looking relay is the fuel pump relay from my 94 vin w) Before spending money on a relay (cheap as they are) I suggest a fuel pressure test if the pressure drops quickly you have a leak somewhere. Also if you can hear the fuel pump when you turn your key on I doubt the relay is your problem.