4 Wheel won't engage

I have a 1999 Silverado Z71 5.7 L. When I push the switch to go into 4 wheel low or hi I can hear it "clunk", but it doesn't engage, the light just starts blinking.

I am sure I know your problem. I really don't know the proper terms. But your front wheel calibrators that shift them into front wheel drive are mostly made of plastic. They tend to short out if the 4x4 is not used liek once a month for about ten miles or so. I too have had the problem. If it is not to late, keep trying to engage. Once engaged drive for about 10 miles. tehn, ofcourse do it once a month and you should have no problems. If it is to late. Well as my Trany mechanic infomred me. Don't take it to a dealer . iT will cost approx. $1200 for them to fix. any transmission mechanic can fix just the needed parts for about $600. Again this is coming from a converstaion I had yesterday with my tranny mechanic.
Hope this helps

I took the truck to the dealer because it is still under a warranty that I bought. They replaced the actuator for the four wheel drive. It would of cost about 300.00 if it wasn't under warranty.

Yeah the actuator is for the four wheel drive. Thats what you usually need to replace if the 4x4 switch just blinks on you due to a short.
I have avoided that by just driving in 4 wheel drive every now and then.