It is important to determine if the vibration occurs in every gear at a certain RPM, if it occurs throughout the RPM range, or if it only occurs in a certain gear.
If it's the first and you hear a ticking sound behind and to the right of you, it is probably the u-joint. If it is, spend the $150 and buy an aluminum drive shaft. It's a great price, easy install, and will gain you a little power.
If it's not at a specific RPM and in all gears, let us know what the complete symptoms are.
I broke my emergency brake cable a few years ago and probably won't replace it until I upgrade to Baer brakes. My guess is that the things are siezed by now anyway.
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351W 89 Mustang GT Convertible