It's either:
The torque converter is locked up causing the engine to stall every time you put it in a drive gear
OR
Something wrong with the engine, totally un-related to the transmission.
Things as: Horribly poor ignition timing/ way off cam timing
How does the engine run when in Park or Neutral? (Keep in mind that running without a load, and running with a load can be very different on the engine)
Will it rev up quickly and all the way to the rev limiter? (
THIS VIDEO should explain what you want it to do)
If the engine doesn't run perfectly out of gear, start with checking the ignition timing. It's more than likely cam/ignition problems than anything else. (jumped timing belt) If it's the torque convertor... Might as well buy a new one, don't go for a rebuild from some crummy shop.
Even better if you like performance. Perfect time for a custom torque convertor and a transmission valve body upgrade! Or a manual swap if that's your thing!