I looked in my factory shop manual and my factory assembly manual. Then I looked at my '48 1/2 ton. It is most definitely not where the e-brake spring attached. The e-brake spring is attached to a crossmember closer to the rear end and then to a tab on the e-brake rod that goes to the rear, and that agrees with the factory assembly manual.
Then I went out and looked at my torque tube. I have the same piece on mine, and it's obvious that nothing was attached to it (no wear marks or signs of something attached.) I don't know what it's there for, but it's definitely *not* for balancing the driveshaft because the outside of the torque tube never turned. The driveshaft was inside the outer tube, so putting "weights" on the outside housing would accomplish nothing.
I'll ask my friend Wally if he knows what it was there for. My best guess is a jack point. If you removed the torque tube assembly (which would include the rear end, driveshaft and both wheels), you could rest the front of the assembly on that point with a jackstand that had a pin in it that would fit through the slot and keep it from rolling off the jackstand.
