I haven't spent much time on this but i did one small test and I'm almost ready to get into it more, so if the following sheds any light let me know what else to per sue.
I took off the vacuum lines at the cad. The red line on the right had vacuum in 2wd and the other one in the left had vacuum in 4wd both had a healthy pull in them.
With those lines disconnected I was able to drive with no noises, clunks etc.
The 4x4 light in the truck didn't come on which is perfectly understandable.
Does the transfer case itself rely on those vacuum lines at the front axle in any way for it to engage?
Was the tcase actually engaged?
I don't think this explains the problem with 4h not seeming to really work but it might explain the clunking coming from the front?
I'm going to plug those lines and try again.
Also the vacuum solenoid that they plug into has a fair amount of play, is that normal?
Thanks

I have a 99 and my 4high or low and my vents work only part time and cruise control does not work at all along with 4 high and low. My actuator works and operates smooth I have ordered the switch for my tc but I cannot find the plastic vaccine lines going to any of these

99flatbed
If the 24 valve is the same as my 12v then the front vacuum lines are on the right side of the axle where there is a bulge called the cad I believe. On the back of that there's a vacuum solenoid that they plug into they come together in a squarish rubber boot, one is red, one is black, you can follow back from there, google 24v cad 4x4. Hope that helps.
Back to what I've been up to.
I decided to go get a free check at a transmission place that said they work on transfer cases.
Nice people but I got what I paid for, nothing. 4 hours they had my truck and they told me the noise was definitely coming from the transfer case and they called the tcase company and they told them that it shouldn't make noise wtf!
They should have bought me lunch.
So today I did an "in place" partial teardown.
The previous owner said that they had recently rebuilt the tcase and now I believe him, it looks new in there. No sign of wear whatsoever, I'll take pictures tomorrow and look very closely at everything.
The only things I found were that the installer was a little over zealous with the rtv application and that ended up obscuring the oil pick up screen and a bunch was at the bottom on the magnet. No sign of grittiness at all though. Also the o ring for the speed sensor had failed but not enough to affect the oil level really.
I now think the linkage was not adjusted correctly but the main problem is up front.
I will ask when I post pictures that you guys look for anything missing or other problems. I'm deep into it now, no looking back!

I put my transfer case back together and drove around for an hour ripping up the yard trying to determine the symptoms accurately.
I tried different approaches to the linkage tension but couldn't improve my situation UNLESS I held on the 4x4 shifter. When I held the shifter down I was able to determine that I can get 4H to work but it pops as well as 4L
I'm back to believing it is in fact the transfer case. The only hint (to me) that there was any wear in there was on the 'Upper Chain (Drive) Sprocket' I'll post a pic.
Also, there was some fine metallic dust on the magnet when I cleaned it up.
I think my next test will be to disconnect the shifter and move the shift lever on the tcase itself, now I have been in there and re-assembled it I can see it is pretty simple and the shift is accomplished by a big cam shaped chunk.
Take a look at the wear on the drive sprocket and let me know if that looks bad.
thanks

I made a video in case anyone would like to see.
This was with the shifter disconnected but the case supposedly in 4L

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How does the speed sensor go back in?
I replaced the broken o ring and shoved it in before I replaced the tcase, I moved the shafts on the tcase and nothing was binding and now I have no speedo.
I may have put the sensor in facing the other direction but I can't see that messing it up. I hope I didn't chew it up :(