More Interstate Trouble

The YJ was much better on the way home.. Ran better, used marginal gas.
However, now in first and second I get a vibration. I also get (as a
bonus) some noises that seem to be coming from the rear end.

My first thought was that something was FUBAR in the tranny, but it
seems to be more relative to speed (only does it below 20) than gear
and RPM. Is this the very last sign that I need a new rear end? Or a
rear end and something else?

Thanks for all the help.
D