84 s10 1.9

POPPING BACK IN THE CARB IS AN INDICATION OF WIRES CROSSED. PUT THE TIMING MARK ON THE CRANK PULLEY ON ZERO AND REMOVE NUMBER ONE PLUG. FEEL THROUGH THE HOLE IF PISTON IS AT THE TOP. ARE THE ROCKER ARMS ON BOTH VALVES ON NUMBER ONE LOOSE AND UNENGAGED(NEUTRAL). THEN REMOVE THE CAP AND OBSERVE WHICH WIRE THE ROTOR BUTTON IS POINTING TO. IT SHOULD BE POINTING TO NUMBER ONE. THEN ADD THE WIRES IN FIRING ORDER AND TRY TO CRANK AGAIN

Sounds like the ignition timing needs to be reset.

Check if the timing advance is working. The vacuum advance on the distributor may be bad. A timing light should show good advance when engine is revved up.

Had an 84 silverado 305 that did the same thing. Try giving it a full tune up. Plugs, wires... Mine turned out to be fouled plugs. I didn't find that out till after i tore it down & put a timing set in it. Wasn't getting good enough spark.

check your cap and rotor button if its a 4.3 they are notorious for going bad

Check if there isn't any blown "engine control" fuse. It looks like the computer isn't adjusting the timing as the engine accelerates. I assume you've already checked for open circuits at the ignition system. If not, do so, making sure all your connections are tight.

When it comes time to change the plugs and wires and distributor cap do the cap and rotor first. Remove the old cap with the wires still attached. Put the new cap and rotor on the distributor and transfer the old wires one at a time. Make sure both caps are lined up in regards to mounting position on the distributor. The cap will only mount one way. When you have this done then you can do the plugs and new wires, again one at a time. This way, when you are done the van should start right up and purr like a kitten. Hope this helps.

Check dist. cap and rotor and replace. Just fixed one last week. Rotor was causing a misfire under load and coil contact to rotor was worn out down too the cap. Graphite dust from the center contact in cap was all over the rotor causing a misfire. $25.00 in parts or less. While you are there replace wires and plugs. Autolite ap25 plugs 2.99 each

there should be 2 clips on from the rotor housing to the cap, pull the clips off and lift the cap, you will want to change the rotor inside as well. do not remove the spark plug wires from the old cap yet...replace the rotor, it will only go on one way, and put new cap on, it too should only go one way. now, one at a time, pull the plug wires and put them on the new cap that is mounted. this will ensure that you have them in the right order.

seems to me there may be a problem with the transmission, because you say it runs fine in neutral and dies when you put it in gear!!!
only logical thing if everything else you mentioned has been ruled out!!!