1991 Chevy Astro rebuilt transmission/replacing engine

Transmission problem
1991 Chevy Astro Automatic

I took my astro van in because I heard a clunking noise in the rear. But ran fine. My mechanic said it was the transmission slipping. He rebuilt the trasnmission. When I got it back, it still had the clunking noise, but then it vibrated horribly when it was in drive at a stop. He then said it needed a new differetial. He replaced that. Got it back, no clunking, but still vibrating and now over heating. (It did not overheat, nor have any vibration initially. ) Took it back, he said it needed a new engine! He put a new engine- with 70,000 miles on it. Got it back tonight and it is still vibrating really bad while in drive. It is really loud when shifitng from reverse to drive. I'm wondering if he rebuilt the trasmission incorrectly, and could that have been the cause of my engine needing to be replaced. I'm now broke, frustrated, and have a worse running car!