It does sound from your description like the "dreaded diff clunk" but that's not too hard to fix and usually doesn't present a problem unless it gets really bad and you abuse the car. For more information on the repair I suggest this "how to" from the Chicago Land MG Club.
http://chicagolandmgclub.com/photos/b_clunk/
'73 Midget (V6)
'59 MGA (I6) under construction
'73 Lotus Europa
"There is a fine line between a hobby and mental illness"