4-wheel-drive trouble on Chevy Colorado

[Red1]

I have a 2004 Chevy Colorado and the 4-wheel-drive won't engage. This happened once before a couple years ago. I had it repaired but seldom used it after the dealership fixed it. Awhile back I tried to go into 4-wheel-drive and again, it wouldn't engage. I took it back to the dealer, but it worked for them. After that, I started "exercising" it about once a week, putting it into 4-wheel-drive and taking it out again. Sometimes it worked and sometimes it didn't. Now, it will not work at all. The light blinks and I hear a whirring sound but it never kicks in.

I'd like to fix it myself instead of giving the chevy dealer more of my money, but I've never worked on 4-wheel-drive before and don't know where to start. I've heard there are 2 different actuators and I don't know how to determine which one is the problem here -- if, in fact, that is the problem. There could be something else wrong as well.

Anyone have any ideas? Thanks in advance.