The service 4wd light has come on in my wifes 2000 Blazer. Truck has 103K on it. The light was coming on and off periodically, now comes on pretty much all the time. The buttons on the dash to select 2 hi, 4 hi, 4 lo, and auto 4wd also don't work correctly. Right now it's in auto 4wd. It doesn't change by simply selecting a single button. You have to hold two of them together to get it to change. I can't seem to get it back into 2 wd now. Have to mess around with putting it in park, neutral, or reverse in order to get it to change. What do you think the problem is and what kind of dollar amount could I expect to pay to get it fixed?

You need to get it on a good transfer case scanner and read out the transfer case code or code and post back code no.
Then someone can help.
The diagnostic fee and get code can run around 50-100$
Sometimes pulling the fuse and reseting it helps.