Speed specific vibration

i have a 1980 kz550 ltd and whenever i am decelerating only in the speed range of 58-65 mph my bike begins to vibrate pretty bad. i notice from looking at my shadow, that when this happens, my rear tire starts to hop up and down at a very fast rate. i have tightened the chain to how tight it should be. i think it might be a worn chain and/or sprocket problem but i'm not sure. any advice would be appreciated. thanks

Your chain may be on it's last legs as tight and loose sports are causing this vibration. Put the bike on the centerstand(if you have one and then spin the rear tire to see what I mean). If you do decide to replace the chain replace both the countershaft(engine or front) sprocket and the rear sprocket as a new chain on old sprockets is as bad asold chain on new sprockets. The shop manual will show you what tools you will need.