WDS, 2WIRE, & Airport Express
2Wire modem/routers evidently don't support WDS protocol for wireless network extension. Nevertheless, I have a 3800HGV-B Gateway provided by ATT-Uverse, whose wireless network I have extended.
The linksys WRE54 works with it to extend the range of the network, regardless of no WDS at the 2Wire end. (I don't know what protocol is being used.)
Airport Express will work wirelessly with the 2Wire as a print/audio server but not to extend the network.
Once the WRE54 is up, Airport Express--which explicitly supports WDS and will not extend the 2Wire base station directly--can be configured "manually" as a "remote" to the WRE54, to extend the network range.
The result is good wireless connection throughout my house, whereas it was spotty without the extensions.