Front Wheels Quit Driving Overnight While Parked!

My 1990 Astro AWD came up my driveway perfectly through 8" of snow making no strange noises of any kind. The next morning when I went to leave it just sat there spinning. I put down the window and tried rocking it back and forth and noticed that the front wheels weren't doing anything, just the rears were spinning. I had someone watch to verify this. I crawled underneath to see if snow had gotten packed in somewhere but, there was none. I don't have enough jacks or jack stands to get all four wheels off of the ground to check wheel hubs or drive shaft. I just don't understand why it would quit working while parked overnight! I have had 4x4s all of my life but, have never run into anything like this. I have searched the web and found nothing that pertains to this problem with a 1990.