I have a 91 PU with a 22RE. It has always ran great, never had a problem at all. ABout a month ago I put a full tank of gas in it, first time since Ive had it. After that I drove it about half a mile to my house. When I tried backing out of the driveway(downhill slope) it died as soon as i put it into reverse. It has done this since. Now whenever the AC is on if i press the gas the tiniest bit even and let go, it dies.

I have no idea where to start looking.
the only recent mods ive done is installing a flowmaster 40 series about 3 weeks ago.

it also has a major exhaust leak at the manifold due to a stripped out bolt hole. could that affect it?