1992 Toyota Pickup Vibration while driving at highway speed

I own a 1992 Toyota pickup and last year a vibration started to appear while driving about 55 to 70 and i have put diff tires on it, rebalanced the driveline, and had the wheels balanced along with an alignment and it still happens, i havent replaced the u joints the whole time i have owned the truck and that is about 5 years and i have put over 90,000 miles on it. Im just wondering if it is the drive shaft or u joints gone bad, i havent noticed any play in the u joints. Please help me its driving me nuts