Just my as every body else has a cure to your problem. You say that you get two runs before the rear joint goes bad? I would think if it was in the trans the front joint would go. I would look at the rear pinion angle , the rear housing is probably sticking straight up. U-joints can take alot of vibes before they wear out. Also look at the rear yoke to see if it's spread out , if it is, it won't center and cause a vibe and even kick out the joint.