There is a scenario where the Direct Drive Bushing & Assembly seizes up - Total Rebuild needed with Hard Parts to fix.
Seizing of the trans enough to cause engine to stall, yet trans performs normal under all other conditions. Interesting.
From the complaint you've described it is important to rule out all possibilities that it is not an engine mechanical, electrical, fuel management problem.
For example: low cylinder compression, weak spark, low fuel pressure, emission control problem.
To confirm trans malfunction described...
What is the idle rpm's at normal operating temp. Drive, and Low? Vehicle not moving.
Does the engine stall when in Park/Neutral, and rpm's are brought up, and quickly letting up off the gas?
Does the trans produce any abnormal sound when engine stalls?
Check trans fluid for low fluid level, dis-coloration, and smell at dipstick.
If dark color, and/or burnt smell, remove trans pan and check for metal particles /clutch material.