[Northkeep] auto advice?

[Kevinkeary at aol.com]

Anybody out there know anything about small 4-wheel drive systems?
 
We own a Trailblazer,'04. We've already replaced the rear differential  
because some previous owner or witless mechanic put the wrong lubricant in it.  
We only use the front drive system when we're moving a heavy trailer around 
our  yard, off pavement.
 
It has acquired a roaring sound that Keigan describes as an airplane roar.  
It is getting louder. It seems to be coming from the forward portion of the 
 vehicle this time, but it's hard to tell; it only shows up above 20mph, 
gets  loudest at 40-50mph and weakens above 65mph. (We drove to Houston and 
back this  weekend.) I suspect a u-joint on the forward driveshaft, but 
crawling under it  this morning, I can turn the forward shaft freely (4wd is not 
engaged, but front  wheels are on ramp and can't turn) and there is no play 
in the u-joint on the  front of the driveshaft. (There isn't one on the other 
end, the end that fits  into the forward transmission or whatever it's 
called.) I can't detect any play  in the front drive axles either.
 
Last Thursday I replaced the brake pads on both front disks. I didn't  
detect any problems that felt like wheel bearings -- and I don't THINK that's  
what this sounds like anyway.
 
Any ideas?
 
Kevin