Service 4WD sounds serious. It could be as simple as fluid but then you have to find out where the fluid went to, as in leak.....
With that said..... Start with the basics, as Fox said, check the transfer case and look for loose, missing, corroded wires, etc......... Leaks, obvious transfer case damage, fluid levels.
It may work physically but until you figure out what is going on with the system I would strongly recommend NOT using the system until the problem is resolved.
I am with sharp in the question, what do you mean by service at oil station, I hope that they did not fill the differentials with the improper servicing as in wrong fluid and caused further damage. I know it should be a longshot for what are supposed to be professionals but these days you never know.