i'll find my manual and get back to you, what version of the blazer does she have?
Edit: it could also be the computer recognizing the ground was wet due to the lack of traction so it switched to 4wheel drive low (high is for snow if i remember).
Tiocfaidh ár lá
Droid X 2.2
rooted & Deodeodexed as of 3/26/11
yet to be rooted...
Rooted since day 1