This is my second attempt to describe my problem but things have changed a bit since my last attempt. Initially I had a load, deep, growling sound, a bit like a card between spokes, it seemed to come from the front passenger area of the truck and was related to road speed. I checked underneath and found the passenger side inner CV joint was shot. During replacement of the CV driveshaft ($59 exchange from O Reilleys) I noticed that the output shaft that sits within the output shaft tube connecting the front diff to the passenger side CV shaft was sitting on the bottom of the tube, you had to pick it up to slide the CV shaft on to it (its a spline not flange fit). It wasn't loose if you tried to pull/push it from the tube, it just sort of dropped at the end if you know what I mean. Is this normal? Has anyone else found this when you replaced the CV driveshaft? I know there is a bearing in the tube and wonder if its broken up.
The new CV driveshaft didn't fix the noise but it did disappear when I switched between 2 and 4 wheel drive back and forth a few times. Alls still good so I will have a look at the vacuum/cable/diff set up, but any advice on the shaft would be appreciated.

98 Blazer LS Auto

 

My parts explosions show a bearing in the outer tube. Not necessarily at the end where the CV shaft attaches, but it should be sufficient to support the shaft. It is quite possible that this is your problem...