Thanks to both of you guys. Funny, I spoke with two tire dealers (Firestone and Good Year). One said no way 3/4 of an ounce would cause vibration at 40 mph, the other balanced the tires because of it. I still have the vibration though.
It could be:
Crude on the inside of the wheel
Worn steering componets
Bad or loose wheel bearing
Try jacking the vehicle up and rotating the tire. Watch for the tread "walking" off to one side and back again. This is belt seperation. It generally causes the steering wheel to tug at slow speeds and a vibration at higher speeds.
If you had a broken belt I would think they could see that when they balanced the tire. Not always though. You don't say what kind of truck but if you have large wheels, I have 20's, then the problem could be the machine. I bought new tires for my Dodge Ram and went back 5 times to have them rebalanced. They even replaced one tire as they thought it may have a broken belt. Reading on the internet I found out that it takes a much larger tire balancer to handle the weight of a 20" wheel. Dodge sent me to a small shop where the guy could do the job easily. If you have 18" or 20" this could be the problem.
Back in the days when they were only 14 and 15" inch wheels, out of balanced tires were felt more likely at 55 mph. If you felt the steering wheel vibrate, then the out of balanced wheel was on the front. If the floorboards vibrated, then it was a rear wheel. I'm with the crowd that thinks that little of wieght would not cause a vibration.