Worn out '70 MGB rear-end?

[Jurgen Hartwig <gt0003a@prism.gatech.edu>]

Hi all, I'm having yet more difficulties with my little lady.  At 50-70
mph, the rear end makes the whole car shake really violently.  Before
this, the rear end made a clunk each time I went from reverse to first
or vice-versa.  I think this is a worn out ring and pinion.  

Am I right?  How difficult is a ring and pinion replacement?  I will
probably tackle this chore sometime in the spring or summer, and I'll
just have to cover my ears to mask the noise.  Ouch!

Jay