Is there a way to manually engage 4hi by disconnecting the electronics? That would help you rule out a mechanical issue.
The only thing I can think of would be the electronic shifter is not working, or for some reason the chain split, and it is not giving power to the front. I have had Jeeps NP231 open to install a 2low kit, Slip Yoke Eliminator, and a teralow 4:1 front half. There really isnt much that can go wrong with these things.