1999 GMC Jimmy ~ 115k miles

Had issue where 4x4 would not engage. Push switch, lights flash to 4 hi, here something try to actuate, an occasional clunk and it goes back into 2 wheel drive.

I looked at the vacuum under the battery cover, don't really know what I was looking at, but did not see any cuts or tears.

Friend tried to replace the TCM with rebuild and with new, same thing.

As of ~1 month ago, saw a decrease in fuel milage. Was getting ~ 22, now at 15-17 mpg. Realized then that I was stuck in 4x4 but button light still displays that I'm in 2wd. Press the switch, same effect as when I was trying to get into 4wd, lights up and goes back to 2wd.

Tried disconnecting battery overnight and reconnected, same thing.

Any suggestions what I should do next?

Other notable: Occasionally, if I try 4x4 on the road, I may get a slight grinding sound. Slow down and it stops.

Any help greatfully appreciated.

Update: I read the how to troubleshoot 4x4 thread. I went out to examine vacuum hoses. I did not find the additional connector as mentioned in the thread.

I found some lines that are dry rotting. I pushed 4 lo, had it flashing and attempted to manipulate these lines. I got back in the vehicle, there were no lights on. Took for a drive, hit 2wd, it flashed, clicked and went back to 4wd then the light went off (no lights). Hit 4 lo, it clicked, went away and no lights. I'm definately in 4wd still.

Thoughts?

If vacuum line, I see two, one that goes between above the wheel well and another that drops down the engine and goes under the vehicle (central location).

What transfer case?
The vacuum stuff lock and unlocks front axel?
But first.
Is front drive shaft locking and unlocking.

The front axle will lock and unlock. After several attempts, usually works best when I first start the car, I can get it to go into and out of 4wd.

I had oil changed in the rear transfer case.

Any grinding noise I hear is coming from what appears to be the front passenger side.

Does the switch have a Auto 4wd button( 3 or 4 button switch)? I am assuming its a 3 button system. In either case, the slight grinding could be a couple of things. It could be that the front diff is not disengaging totally and is causing the gears to grind, or, it could be that there is a possible cv axle break, causing the front to not properly lock and/or unlock.

I am more concerned about the lights on the swtich going out . Is there a light always on? Either in 4wd/2wd, or whatever. Do all lights go out? Have you looked at the grounds and connections at the TCCM? The transfer case encoder motor can and do fail and can cause all sorts of problems.

Have had all the lights out twice recently. They do come back after a few minutes.

Have not checked the ground, but did change out TCCM with no luck.

3 selector switch. 2wd, 4hi 4lo only options (no traction ctl).

I would wager the front axle is not disengauging. When it does disengage, I can hear it.