bad vibrations in 4wd

ok I need some help on this one,I have a 89 cherokee with a 5speed puguot transmission and an np231 transfer case.tonight when I got my first chance to drive in a little snow I pulled it into 4wd and as soon as I started to get going I noticed one big bad vibration it almost feels as if the whole trans and t case is shaking.its not doing this at real slow speed under like 10 mph?I'm going to go over the front drive assembly and t case in the morning before work.I'm also going to check the transmission mount. any suggestions or help would be greatly aprecatied. I just got my jeep on the road it goes in for inspection on thursday,and this won't fail it for inspection but sure does make me upset considering that we might be getting some real snow in the near future and a 2wd xj would be useless to me then.Baldwin p.s. I did search but did not find anything more than what i'm going to do in the morning, just thought maybe some one had a problem like this before and might be able to help me out.