Duff,
Yes, 84 is close to the danger zone. You could do a water change......assuming the "new" water is cooler than that in your tank. You could do what Acurasquirrel suggested and place a frozen zip-lock bag in your sump. Leave any windows near your tank open at night to achieve maximum cooling. Reduce your lighting time....but if you have corals, don't reduce too much all at once or you will shock them.
I feel your pain. My tank was at 82 last night when I got home but I left windows open and it's back down to 78 this morning. I think tonight is time for the air-conditioning in my house.
Good luck!!
Jim