4WD not engaging

[Cary Coleman]

I have an 04 Tahoe, LT 5.3, Vin Z with the 4 button Slector on the dash. My 4WD is not engaging in any mode. I bought the vehicle used and noticed at night there was no dash illumination on the selectors so thinking that I replaced that unit, however, I still cannot get the 4wd to engage, though I can at least see the selector switches in the dark now. Before I start further troubleshooting, what is the logical progression for proceeding? It's not throwing any codes pertaining to the transfer case or related parts.

Thanks in advance