Nitpicking...but for accuracy's sake...2003-2007 got variants of the A750F (the 5 indicates 5 speeds).
What's the word, @Jacob117? Is one of your CVs spinning when in gear?
This one is interesting as the 343 is a pretty venerable transmission. I have no idea how much you know about these trucks, so don't take it personally here. We need to rule this one out for certain:
When you hit the center diff lock button, do you hear a faintly audible click from near your...