1997 Oldsmobile Bravada Front Axle Low on Gear Oil, Whining

My 97 Bravada has been making a whining sound that occurs above about 30-35 MPH and continues on into the 80+ MPH range. The sound stops when accelerating, but as soon as speed is maintained or deceleration occurs, the whine comes back. I checked and replaced the rear differential oil (A Gov-Lock Posi-traction differential) and nothing changed. The gears looked fine but there was considerable play in the drive shaft and yoke which was corrected by replacing both original rear U-Joints. The sound still existed, but a shimmy on acceleration was corrected. I then checked the front differential fluid and it was low, it took about 3/4 of a quart. The noise quieted down, but it still exists. My question is: If I see no fluid leaking, where would it go, and, What would an approximate cost be for repairing this problem? Thanks, Nick.