Steering wheel vibration

I was just wondering if I have a major problem or not? I have a 1994 C/K 5.7L pickup about 129,000 miles and I haven't used the 4 wheel drive in a while (about a year) so the other day when it snowed I shifted into 4 wheel and at 10-15 mph I got a vibration I never had before. Is it not fully engaged? Diff fluid changed almost a year ago. Luckily it disengages for me. Just wondering if thiers a big issue here or is it just froze up from lack of use? Local temps around 0 degrees. :banghead: Appreciate anyones opinion. THX:banghead: