so assuming it's not a low idle problem and it's quitting because of the normal load of the trans, the problem would almost have to be in the torque. whether the lockup clutch, or a failure causing it to bind, that's the only thing that's turning when you're stopped at idle in gear.
If that tranny works like most I've fooled with(heavy equipment, mostly), the input shaft spins freely in park/neutral(no oil pressure on any clutch packs). This would allow the TC to spin as one unit, as it does in lockup mode.
When you engage the clutches for a gear, this locks up parts of the planetary gearsets, thus creating a solid connection through the gearsets. When stopped, the impeller of your torque turns with the crankshaft, and your rotor stays with your trans. input shaft(which spins freely in P/N). If your torque is locked up, it could kill the engine when you put it in gear. However, it should feel harsh when it does.
So if you could get the car rolling 5-10 mph, or perhaps VERY SECURELY raise the drive wheels, and put it in gear then, you might be able to try it. Do that, then hold the brake and stop the wheels...if the trans is causing the engine to quit you should be able to listen and tell. Hope this helps, anyone feel free to correct me if I'm wrong.