they r not that sensitive if the object reflecting the wave itself is moving.... since sound wave speed isnt that high... maybe using reflection could be sensitive enough....
Use MTI algorithm (Moving Target Indicator) to detect moving object. (search Google for: MTI and "Moving Target Indication"
If you use light signaling then these Time-to-Digital and Digital-to-Time converters can be used to range the moving object: http://www.acam.de/index.php?id=26&L=0).