My 90 5.7 runs OK at idle in park or netural, but dies at idle when in gear.
It gets so bad at times I have to double foot the gas and brake to keep the truck running at stop lights and in traffic.
I have changed EGR, MAP, PCM, TPS, IAC, PCV, fuel pump and filter, and checked for (but not in great depth) for vaccum leaks.
All this has occured in the last year. Some of the part replacements I have tried over the last year have seemed to help but never fix the overall problem. I also change the fuel filter with every oil change (@3000mi) as well as spray the TBI out with carb/FI cleaner and add injector cleaner to my gas.
I am out of ideas, as well as patience and money.
What could it be? Vaccum? Fuel? Ignition? Electronic?

Welcome to AF. It idles so low it stalls, how do you restart it? Is this what is happening or it gradually dies, is it instant? Have you scanned it for iac counts? Any codes?

It idles fine in park or netural, but when you put it in gear, the load of the drive train seems to put a load on the system and it stalls. Then when I take it out of gear, it starts right back up. Also when I come to a stop in traffic, the engine will starve then stall. This is not instant, it will slowly starve then stall, @ 1-2 sec.
I do not have the equipment to check for IAC counts. I am not aware of any fault codes. The check engine light has not come on since I replaced the EGR valve.

Sounds like your fuel pump is failing or there is carbon tracing in the distributor cap. I'd start with cap/rotor and wires and then replace the fuel pump.

At one time I had a 91 5.7 that was doing the same thing and it turned out to be a cracked injector.