4 wheel drive doesn't work

I have a 2002 GMC Sonoma ZR2 four wheel drive. If you are moving in drive and push the 4 wheel hi button nothing happens. If the truck is in neutral and you push the 4 wheel hi button, 4 wheel high and 4 wheel lo will flash a few times and then it goes back to 2 wheel hi. I replaced the dash buttons but this didn't help. The problem can be temporaily fixed by removing both battery cables for about 30 minutes. I would appreciate any help you could pass my way.