Drive shaft? can you explain a little more why you are thinking drive shaft? From the rest of your description I would think the problem would be steering related; i.e. strut mounts.
how many miles are on this beast?
Sounds amazingly like an issue I had with my first engine...
Does it eat any oil between changes? There may have been an incident that hurt the timing components before your ownership. I'd suggest you check/ have checked the timing components.