I would try jacking the rear end and placing it on secure stands, then try placing it in gear at idle. If the converter is locked, it should spin the tires freely and not stall. If it starts to spin and then stalls, maybe remove the rear wheels and try again.

Another thing you might want to check, (and this is way out there), but just maybe your distributor is wired in such a way so as to supply power only when in park.