My son is now the proud owner of my old 89 Chevy 2500 4X4. It has a TH400 and a 350, which have both been rebuilt. It has ~ 80k on it and has always ran very strong with zero problems... Until recently.

A couple of weeks ago he called and told me his truck was running very poorly and that it had run fine the day before.

It started out looking like a fuel problem but no so sure now. I can get it to run and idle fair IF I unplug the MAP sensor. If unplug the MAP and put it into gear, it will die.
BUT, if I leave the MAP unplugged (Vacuum line only) AND disconnect the TPS, it will continue to idle once I put it in gear.

I have replaced/inspected so far:

Fuel filter
TPS sensor
IAC
Distributor(complete including rotor and cap)
MAP sensor tested (I may have diagnosed incorrectly, however)
Plugs and wires
EGR is working

As noted, I can get it to run smoothly in park if I unhook the vacuum to the MAP but if I put it in gear it will bog down and die.

AND, if I unplug the MAP AND disconnect the TPS, I can put it in gear and it will continue to run. To the point I am pretty sure that I could drive it. But if I plug either of them back in, it will die.

I could use any input on this one.

Thanks in advance.