I have a 2013 Nissan Frontier, had the engine mounts replaced at about 70,000 miles because when the AC cycled on there was a very bad vibration. The dealer told me it was a broken engine mount, it was under warranty so I had it replaced and the problem was fixed. I now have 136,000 miles on it and the vibration started to return. No longer under warranty I replaced the left and right engine isolators myself and now the vibration is much worse. There is an upper mounting bracket that is between the engine and the rubber isolator, they felt solid when I replaced the isolators but could one be cracked causing the vibration or could it be the transmission mount ?