I would have your mechanic check the rear drive shaft. I went through the same issue and as everything else was ruled out my mechanic said it must be the universal. Sure enough when he removed the rear drveshaft and we saw it together the universal is free in one direction and slightly binds in the other. (Note slightly). Another good item to check is the transmission mount. I have not cured the problem on my car. Learnt to live with it and be less sensitive.