03 Tahoe Transfer case issue

[Paulio91184]

Hello all. First time poster here. The last time I pulled my boat from the water the ramp was muddy and i had rear wheel spin. I put it in 4wd high and nothing, rear wheels just kept spinning. However, auto 4wd worked just fine and hauled the boat right out and had great traction. I've since tested it about 10 times and its always the same result. Auto 4wd works and regular 4wd hi doesn't. Low doesn't work either. My mechanic had it on the lift and said it was working while in the air but I'm still having the issue. Any help would be appreciated