OK. It's been a bit warm in Dallas and I don't have an air conditioned garage so it has taken me awhile to brave the elements and work on this problem.
I went the easy route first and replaced the differential mount and associated rubber but, alas, the rumble is still with me.
I'm leaning towards Mike D.'s thoughts at this point (drive shaft center support bearing and rubber mount). I may wait until we get a string of 80's before tackling this so it might be October but I will report back.