Service 4x4 light on help!
 

I just had the Service 4x4 light come on in my 99 Yukon Denali. Any tips or tricks to help troubleshoot this. It wasn't on this morning but I did drive through a deep puddle/lake. Also what kind of ATF do I use to service the t-case and how many quarts. I have had the truck 4 months and it doesn't have a manual. Thanks.