I just got my truck back from some extensive work and now the 4HI and Auto 4WD isn't working. When I press the Auto 4WD button the light just flashes, when I press 4HI the 2HI light goes out but the 4HI light never flashes and it never engages.

Any chance something was left unplugged? They did a rear output seal on the transfer case along with a whole bunch of other stuff.

 

I would probably take it back and ask mechanic.
Maybe, like you said, simple as something left unplugged.

 

What happens when you try to engage 4LO? It is doubtful that something was just left unhooked. A scan of the TCCM to retrieve any stored codes could give you direction.