The idle vibration could be from a marginal spark plug, a marginal
ignition wire, dirty fuel injector, etc etc. Adding a can of fuel
injector cleaner to the fuel is a painless start and eliminates the fuel
injector possibility. After that it's $$.
Tires have a static and a dynamic balance, at 80 mph the dynamic balance
is the issue. Have a different tire shop check the tires for dynamic
(high speed) balance.