[bobcowan]

Generally speaking, when tires or drive shafts are out of balance the vibration is speed related. When engines, clutches, and transmissions are out of balance it's RPM related. Generally speaking.

When they're load related, look for things that are broken or loose - U-joints, transmission mounts, motor mounts, etc. Dodge trucks are famous for this kind of vibration with bad U-joints.

Also look for things that change relationship under a load. Like a rear suspension that gets compressed, or a front suspension that isn't. That also includes motor, transmission, and differential mounts. Try using a crowbar or jack to move some of those components around.