Stalling when transmission is shifted into reverse

More then likely the reverse clutch is getting applied or not applied properly when you are putting it in reverse so when you do it is not applying the reverse correctly and it stalls the engine.

Unfortunately this is pretty common on Ford transmissions as they use a servo that applies a band for reverse. I attached the process below on how to go through this and make sure it is adjusted properly or replace it as needed.

Also, you will want to start with checking the fluid just to be sure this is not the issue. It it unlikely to affect only reverse but if that is not correct let's fill the fluid first and then go from there.