gardner:
I haven't tried, but it looks to me like the sensors on a Parallax Ping or equivalent could be de-soldered with some care. If it were my project, I would try wiring the transmit transducer on a length of twisted pair and set the two up facing one-another. The one I have handy is a Seeedstudio unit, and it has the T and R transducers conveniently labeled. They run about $14, so it's not a huge investment to experiment.
In place of the twisted-pair wire you could use a radio link. See this example on The Ben Heck Show: