Engine shuts off when transmission is shifted into gear?

Yep, this happens when the torque converter falls apart and locks up the engine to the transmission, so when you put in gear it stalls the engine much like a stick shift car when you let the clutch out too fast. It could also be a vacuum leak which this guide can help find, test it with the engine cold:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

and

https://www.2carpros.com/articles/stall-at-idle

If you can shoot a quick video so i can see the engine stall, I can tell for sure. You might want to try holding the idle up a little when you put it in gear, if the truck tries to take off you know it is the converter. Let me know what you find.