Vibration under load

If it's speed related (IOW, if the vibration varies with road speed, and not with engine speed or gear changes), then the vibration is probably from something behind the tranny output. So the u-joints are a decent suspect, and the motor mounts aren't. The u-joints would be a really good place to start, as they are easily accessable, and often fail in that mode. From there, the options can be axle/rear end problems, wheel problems, suspension problems (not necessarilly in that order). Good luck.