Tee into the black line going to the transmission modulator and go for a drive. At idle and not moving, you should have somewhere around 12-15" of vacuum going to the modulator. Under strong acceleration it should drop to ~5" and at WOT it should drop to 0". If you have good vacuum going to the modulator, there's not a lot you can do but live with it. Rebuilding a differential is not for the faint of heart, it would be a lot easier to source a replacement or have yours rebuilt professionally, especially if the gears are worn.