what could cause heavy vibration under load or not under load

it started as a occassional miss in the engine when in overdrive only. would have to shift down to 3 for it to quit. Then replaced the distributor cap, rotor, plugwires & sparkplugs (which are burning almost pure white). This helped, but now (more often) at lower speeds (and occassionally around 45-60mph) the truck vibrates and shakes vigoursly and begins to loose power and will barely climb a hill. As the truck is run longer with this vibration, it seems to heat up the transmission to the point that fluid blows out a vent tube. The truck does not overheat watching the temp. guage; the check enging light does not and never has come on; and no antifreeze seems to be coming out the exhaust pipe to indicate a gasket leak. The driveshaft and U-joints are not loose as well.