Engines shuts down after selecting gear from "park" to "drive" or "reverse"

What is your question?

I have a 1996 Indy Ram pace truck that sits most of the time (I use a battery switch as the truck has a minor battery drain due to what I believe is a short in the radio). Lately, when I start the motor, I have to keep pressure on the gas to keep it running. When I try to put it in Drive or Reverse, the engine dies...and it does so without a sputter or knock...just simply goes quiet. If I want to move the truck in the driveway, in order to keep the engine from stalling I need to keep pressue on the gas pedal, and my other foot on the brake. Unfortunately I do not have a code reader, perhaps I should purchase one and try that, but does anyone have any ideas on what's happening? Regards, Joe