Is a Transmission Flush needed at 84000 miles.

Yes it is necesary. It is called Preventive Maintenance. In fact i would done that mutch earlier.Like your engine needs oilchange all others systems that use oil or fluid the fluid needs to be replaced .Including your brake system.

Make sure whoever does the service uses BG chemicals to flush.

i agree with both guys, preventive maintenance is the answer before there is any issues. i would follow thw dealer recomendations. Roy