Homerdan

I have recently purchased a 2001 astro awd van. Noticed a crabbing shudder when tightly cornering. Flushed the rear end and transfer case and installed the special blue lubricant chevy specifies. But still all I have is rear wheel drive. Shuddering has gone away and Im glad of that, but I live in Alaska and bought the van to have 4wd in the snow. I got stuck in sand the other day and only the rear wheels engage. How can I trouble shoot the awd system to isolate the problem? This is my first post so I hope for some help. Tks Dan