Don't no if this has any merit as it involves a completely different vehicle but the problem sounds identical to my DW's Honda Odyssey. The dealer was stump and dumbfounded, they even replaced the mini vans little trans axle and this only worsened the vibration. We were about to apply for NJ lemon law as the thing was in the shop at least 6 times and then we get this recall notice in mail for our model year Odyssey, 2007 at the time, the recall for a shuddering transmission issue, we promptly brought in and they fixed it in 15 minutes. The vibration was completely gone. Apparently the issue involved a computerized setting that regulated the trans axle clutching.
Not saying this is anything remote to your issue but perhaps the problem may lie in the tranny.
