My 1988 gmc 1 ton has a nasty vibration while driving down the road. It gets worse with speed. At first i thought it was the u-joints, but I replaced them and its still there. The vibration goes away when I pull the truck out of gear and let it coast. Also the vibration isn't constant but seems to be rythmatic with the engine rpms. Is it possible the balancer could be going? How do I check it? Engine only has 50000 miles on it.