2003 Avalanche Z71 5.3. I drove through some snowy conditions, did so in Auto 4x4, no problems. I get to a stop light and stop. Light turns green, I hit the gas and the trans slips badly. I go about 300yds, and pull over. I manually shift through the gears, and I have none. No forward gears or reverse. I shut off the truck and restarted it, put the truck in gear and it was fine. Ran through all of the gears perfectly, all the way up to 60mph with no issues. I get home, park the truck. Check the tranny fluid and it is full, clean, nice red color and does not smell burnt. The next day, I start the truck and I have no gears at all. Just revs free as if in neutral or park. The shifter indicator in the dash shows that I am shifting through all gears but it wont even try to move and the idle speed doesn't drop like it is even going in gear. Is it possible that the snow has frozen a vacum line or something? I have no idea where to look and don't understand why it slipped then was fine after restarting. I can start it, put it in reverse and it's fine, go to drive, it's fine, go back to reverse and I've got nothing again. Then restart and it works.