After checking the halfshaft issues that John suggests, try tightening ALL 16 halfshaft bolts! We've found that even one bolt only partially tightened can cause a severe vibration around 3500. So far, three owners have found and cured a vibration by simply tightening the bolts. And I don't mean 300 ft-lbs; just tighten them all to the same torque. One car developed such a vibration without any work having been done on it. I found a cracked lock-washer (over half was gone). Adding a new washer to that one bolt, retightened, and magically, the vibes went away!
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