Fore and aft vibrations in the rig

Of course it is hard to tell what is going on without experienceing it, but from your description you may be experiencing "pumping" which would certianly be percieved as a heavy vibration. I would check to see that you have adequate backstay tension to keep your mast "sprung" and not moving fore and aft. Because you only have single (in-line) lowers, you need to have adequate tension on the rig fore and aft to keep it from moving. An inner forestay, coupled with good backstay tension, can negate the pumping, IF that is the problem. Be sure that your rig is tuned correctly as well. The basic idea is that you want the rig to be gently curved, the inner forestay pulling the center of the mast forward and the backstay and mast step holding the top & bottom aft. Your uppers & lowers hold it side to side. Backstay tension in this case plays the role of keeping the mast from moving, and also allows you to control the sail shape, to a small extent. As your wind picks up, so will the tendancy for the mast to pump. So you increase your backstay to both keep the rig adequately tensioned, and flatten your main to depower it... I'm reaching the limit of my ability to explain this, so someone else - jump in!