90 Jimmy 4wd problems

Well we got our first real snow storm on Sunday night and i took the Jimmy out in it, at first the 4 wd seemed to be working and then it just stopped. I pulled in a parking lot and made my girl get out and see if the front tires were spining not the left or right in 4 wheel high or 4 wheel low. I've heard that this jimmy has a vacuum 4 wd switch or something like that. If anyone has any ideas please let me know.
its a 1990 2dr GMC Jimmy with a 4.3 and its automatic with 4wd.

Thanks

Check the vacuum switch on the transfer case if you have manual 4x4 and also check for loose or cracked vacuum hoses.