Since the subframe bushings are the most difficult to replace of the several possibilities for the clunk, I would start by checking the rear calipers and rear suspension components. This video will help, wait until he gets past the transmission stuff.
https://www.youtube.com/watch?v=HkpuYnaeWUg
You can usually see cracks in the rubber around the bolts if the giubo is bad. This video will give you an idea of the scope of the job.
https://www.youtube.com/watch?v=4yCxhyTlysw
It is difficult to diagnose if the problem is the subframe bushings. Usually if the subframe bushing are bad you will hear a cluck when you go over some like a speed bump as well as when you move forward or back. I've attached a video on replacing subframe bushing. I recommended bending the muffler bracket out of the way rather than cutting it off.
https://www.youtube.com/watch?v=haKecoxhWsk