Hey Jessica. Long time, no talk to. Anyways, with the information you've provided, my first step would be an alignment. It sounds like there is a toe setting out of wack from the sound of it. Have you noticed any abnormal tire wear, specifically the inside or outside tread wearing more than the other?
The possibilities are endless, but if the alignment is good, it may not be stable due to bad wheel bearings, balljoints, or tierods.
As far as the unresponsive rack, it sounds like it's developed a "dead spot" in it. It's fairly common with our racks. Only fix is to replace it (rebuilding a rack is a PITA and usually not cheap).
Dan,
The steering shaft might be the problem but only if the rag joint is shot (the rubber plate mounted in the stock shaft). If the rag joint is good and the universal joints are in good shape, the solid shaft is going to make your problem worse (more vibration to the steering wheel).