when i press the 4hi switch the light just blinks.

I'm not very familiar with the Silverado but for test procedures straight out of the Chevy workshop manual there is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online

The 4wd module goes out on those vehicles frequently, it is located on the side of the transfer case.