Speedo Correction

No speed sensor on a '92 Mustang. They have mechanical speedometers. What gears do you have and what size tires are you running?

You'll need to change a gear on the speedo cable where it plugs into the tailshaft of the transmission.

Vibration should not exist. It's possible teh drive shaft is spinning fast enough to amplify a slight imbalence or maybe a tire has a slight imbalence you never felt before. Yes an aluminum drive shaft will help, but it'd only mask the cause if it's a slight tire/wheel imbalence. Make sure they are "zeroed" and go from their to the driveshaft replacement.