Could be a few different things. The thread linked to below explains how this can occur (don't let the title fool you though):
4wd Front Axle Clicking When in 2HI - Goes Away when in 4HI/4LO

You need to check out the cable and make sure that it moves somewhat freely. You will be pulling against the spring inside the front axle and it should return smoothly. If the cable is stiff, you probably will want to disconnect it at the axle and lubricate it from both ends while working it back and forth.

You need to make sure that there is no vacuum running to the actuator under the battery when not in one of the 4wd modes (4auto, 4hi, or 4lo).

 

Thanks so much for that quick reply, lot of great info on that link, and I will check all of this today. By the way, on the vacuum switch on the fire wall, there is a small hole on top of it. Before I switched the vacuum hoses , I had air sucking through that hole when in 2hi. Was that correct? Also, I'm pretty good with my hands, but new to all of this still. Managed to do a lot of my own work so far, but should I bring this to a mechanic to check out the cable ? Just a little weary about disconnecting things from the transfer case.

 

If the hoses were switched, that makes perfect sense. The solenoid valve switches the output from vented to manifold vacuum when in a 4wd mode. If the vacuum line was on the output of the solenoid, it would be vented when it would normally be sealed and the line to the actuator would hold vacuum (depending on the rest of the system integrity - line from the solenoid to the actuator and the actuator itself).

Checking the cable does require that you pull the battery and battery tray, but it isn't that difficult for someone with a basic socket set to accomplish.

 

roger that, today when i got home from work, i messed with the cable, on the actuator plunger side only. It seemed to move pretty freely, like you said, 1/2" to 3/4". but still something doesnt seem right. completely removed the plunger, pushed to cable toward the transfer case and took it for a test drive like that.( with no plunger) it was better,(the vibration) but not gone. I think I will have my mechanic check out that spring, inside the axle shaft. I told him about this forum and he was amazed. If my cable seems to moving freely, my plunger is in good condition, I have switch operated vacuum to it, I believe the problem must now lie in the transfer case. Maybe the 4wd solenoid itself. It seems as though the problem is not getting into 4wd, but its getting out of. It seems to get out of 4wd at the wheels, but not completely in the tranfer case itself.. does that make any sense? I totally appreciate all of this advice. I bought this truck from a mechanic thinking I was gettting a great reliable truck that just needed tires. Man was I wrong, and totally let down. Things are tight and the last thing i wanna keep doin is dumping money into a guessing game. By the way, I noticed when I had everything together and switching at highway speeds, the vibrations seemed less in 4wd auto, does that have any relevance?

 

4auto locks in the front axle, or at least it should lock in the front axle.

How did the shift fork feel inside the axle tube? Could you feel the spring pressure pushing it back?

If the front axle was partially engaged for too long, the splines will be worn and it probably won't properly engage again. The pictures in the link I previously provided show what I'm talking about. When the splines wear, metal gets peened over the edge.

 

Ok, I'm at work right now , but on my way here the vibration was back. ( I had the vacuum line between actuator switch and actuator disconnected at the switch ) if I remember correctly, the cable was not returning on its own under no vacuum , ill have to double check. It's now my understanding that this cable is , let's say vacuum applied and spring released. Would that be correct? I'm bringing to my mechanic, and I'm gonna have him check out these springs, the axle shaft etc., after 6 years in the marines, I like to get my hands dirty, but I know when to call on the pro's when I'm over my head, just tough cause I like this stuff . At least I believe we have this terrible vibration centralized.

 

Just picked it up from the mechanic. He says that in fact, my 4wd is not working. Seemed like it was because on the lift, both front wheels spin, but when he holds the driver side tire, no spin. Sending me to a trAnny guy......

 

Hello,im new to this forum and am having the same kind of problem with a 1998 blazer i just bought for 1200 bucks. however i noticed on mine that the front tires on it are two different sizes? im thinking this might be causeing the problem.when i drive it it will get a sudden vibration in the front end and when i let of the gas it will stop for a few miles then it will start doing it again.if any one can help let me know.thanks

 

Yea, brother, thats more than likely your problem. Or at least I would start there. Matching tires, that are balanced.....

 

just for clarification is my driver side front axle supposed to be locked when in front 4 wheel drive ? is it possible that all this vibration is now due to bad front axles being that they were engaged for so long ? I noticed on the highway when the weight of the truck is on the driver side I get the vibration worse with an additional clicking noise .I see that you say the partially engaged axel will cause worn spines . remember my hubs were engaged for so long do to the vacuum hoses being reversed . maybe bad axle and or cv joints. also my inner seals on my axles are spitting out a tremendous amount of grease,but no tears in the boots