I just recently had similiar symptoms as you describe with my 78 GT. Occasionally the exhaust would rattle - usually far back around the muffler, but sometimes it sounded like it was coming from underneath the car further forward and the sound was less "tinny". Replaced the transmission mount and the rattling is gone, and there is noticeably less vibration when driving in all gears/speeds. It's an easy job and the mount is less than ten bucks.

My OD was like yours (like most!), worked well when cold then usually didn't work at all when car was warm. I drained and replaced fluid twice with regular Type-F (worked better initially then same old symptoms) - then I did it again but substituted a quart of Type-F with Lucas Transmission Fix. The stuff plain worked. I have OD again "on demand". I can drive 75mph on the highway at 2900 rpms, disengaging and re-engaging OD as necessary.
I use a hand-pump to add trans fluid, and it's a PITA with the Lucas because its really thick, but (so far) the job was very much worth it.