1987 Chevy S-10 Makes Noise at High Speed

At speeds above 60 MPH my truck makes whirring noises (down below, I can't pinpoint the noise). When coasting with no added power there is no noise. The noise is worst just when I apply power. When I apply full-power, the noise is there, but not as loud. I'm not sure if this is a minor problem, or a sign of a major problem about to occur. The truck performs well despite the noise. At street speeds, it doesn't make noises unusual for an old truck.

Here's what I've tried:
The transmission fluid level is okay. I have replaced all three driveshaft U-Joints and the Center-support bearing because this is where it sounds like the noise was coming from. This had no effect.

I opened up the differential and removed the axels to inspect the rear wheel bearings. I saw no-damage to anything, so I re-greased the bearings and replaced the seals. I then closed off the differential and filled it to the proper level. The differential looked good. This had no effect either.

I suspect the speedometer cable but I can't be sure it's the problem. I do see the indicated speed fluctuating. I haven't yet removed the cable because I don't have a replacment. I'm not sure how to replace it either.