ok here we go. I have a 98 4 door blazer. 165k on it and a couple of days ago I picked a small clicking noise from 20 through approx 35 mph regardless of rpm's on the motor.Over the last couple of days the the noise has gotten louder and happens at almost all speeds and is even louder after the truck shifts into second from first. Its a continous noise unless I take my foot off the gas. At that point it goes away until I get back into the gas again. I thought it was the U joints in the drive shaft so I pulled it. Nothing. All the U joints felt strong and I checked 2 of the end caps and the needle bearings where still greased and in good condition. I also checked the rear end and found it was down a little fluid so I topped that off. Problem still exsists. I checked the wheel bearing and those seem ok also. Usually when this happens its the U joints but this time I am stumped

 

Clicking is usually indicative of a bad CV axle boot that's leaking grease and allows for debris to get into the CV axle. Any signs of grease being thrown around under your truck?

Here's a clicking noise thread that turned out to be something different than what the OP expected.

Here's another that had to do with a vacuum switch partially engaging the 4WD.

Also, don't rule out the front driveshaft. If I remember correctly it spins even if not in 4WD, it just doesn't engage until you push the button. Have you checked u-joints on front driveshaft?

 

Well its sad to say I bought my girl to my local chevy dealership. I got the diagnosis and its not good. She is terminal. She has a bad rear end. I dont think I can go ahead and spend around $1000 to get it fixed. Besides I also needtwo O2 sensors and a right front wheel bearing. I am also worried about her tranny. So to make a long story short I had to trade her in for a 02 blazer. Its the same rig as the one I traded in so I guess its a good thing just4 yrs newer and about one hundered thousands mile younger. I am going to miss her