Five choices:

Shifter no longer connected to the transmission - linkage fell apart.
(Most likely possibility, IMHO, and, if so, this one is your X-Mas present.)

Transmission's pump is dead so there's no internal pressure to tighten
the transmission's clutches and bands etc.

Transmission dry - no fluid. (Unlikely given that you just drove it)

Driveshaft is broken. (Possible but unlikely)

Rear axle is broken - driveshaft spins but differential allows rotation
to go the broken axle. (Unlikely but possible.)

Jerry,
  I agree -- chances are there is a problem with the linkage. Usually these
re-built trannys are bench tested and are pretty reliable.  Recently, my 420
decided it would no longer go into park. I opened up the side inspection
port under the tunnel carpet and discovered the clamp holding the outer
sheath of the cable had come loose. Everytime I tried to put the tranny into
gear, the entire cable was moving instead of the lever on the transmission.
Probably just something not tightened upon re-install that took a couple
months to loosen to the point you now have trouble.

Webserve

Cross out "no problems" in the paragraph above.

Sorry if I've missed a previous reply - but the first think I'd look at
is the linkage between the lever and the gearbox.
I have heard reports that the clevis pin attaching the lever to the
linkage is particularly liable to break - especially if pressure needs
to be applied (for example to remove the car from Park on a hill).