1998 F-150 XLT 4x4, 4.6L engine, auto tranny.

Everything was fine yesterday as far as I could tell. I even backed up to get into a parking space... however as of this morning, as soon as I put the truck into reverse (you can hear the tranny engage) the truck stalls. Drive and Neutral gears all work fine. Tried putting the truck into 4x4 to see if that would help, nope same issue...

Anyone got any ideas?