Vibration at 85mph...

Yeah... its called worn suspension parts old cars are notorious for that. Seriously tho I would check your hubs, Then struts/shocks, control arms tie rods etc As I am sure you have already checked your axles and have had your alignment done?

I do need an alignment and swaybar bushings. Could they be the cause?

yeah... that solved most of my problems in my 86' shelby charger

Check the inner CV's....that caused my vibration...

balance wheels? you would be suprised how much difference it makes

In my experience it's usually an unbalanced wheel or a damaged tire.

The axles were replaced a little while ago. I think the tires were balanced, but not totally sure.

wheel weights can fly off in a mile, a slipped belt, choppy, or flatsided tire could do it too

I will check those out...THANKS