Truck drove fine this morning. I had to put it in 4X4 to get out of a muddy field.
Now it runs good in 4X4 , but when I switch it back to 2X4, at low speed, it jerks and lunges. Goes in reverse and drives fine, goes forward in 4X4 fine. WTF?

 

Doesn't sound like your 4wd is completely disengaging. How much mud were you stuck in? Did you look under the truck and see if you tore anything up?

 

not much mud,just surface mud, slick. Didn't have to drive hard to get out, just came out easy. That's kinda what I was thinking, that maybe it wasn't disengaging completely, but I'm not sure.