98 Ford, only goes into 4 wheel sometimes

Hey, just picked up a 98 ford f 150 4 X 4. We drove it and couldn't tell if the 4 X 4 was working. Put it up on jack stands, and when you switch it over, sometimes it works, others you can hear it laboring, but not engaging. Not quite a grind but close. And ideas?
Thanks
Neil

I think the front axle itself has an electric disconnect. If it is not working, the transfer case output can't power the front wheels because the differential won't let it....

One up that one...
It is also vacuum activated, not electronic, so the accuator itself may not be bad, check for vacuum leaks.

there is a vacume servo on the right hand side of the diff make sure that is lubed they hang up it gets it vac from a swtch on the fire wall