Checked gear linkage under the shifter, all the way back and into the
transmission - all looked good, no loose bolts.  Called my mechanic, he said
to climb under the back and shake the axil on either side - that was it!  He
installed new CV joints a coupla months ago and the axil disconnected from
one of the wheels.  He's on his way over to fix it (a mechanic that makes
house calls!).

Thanks for everybody's help!

deb