I tried to do the same thing with one base and 4 remote sensor with XBee on all of them, with no common clock signal. I got all of the nodes talking to one another and all was happy on the network. My times were not making sense and one of the reasons was that the packets coming out from the XBee is not deterministic and that makes the latency different with each event from the sensors. I found that I could get time resolution of maybe 0.050 seconds but would be happier saying it is 0.10 seconds.
My timing resolution needed to be about a millisecond so I needed some kind of time base that was common to all nodes and the base so the remote sensors could report the time of the event referenced to that common time base. The time base I am implementing now is using the pulse per second (PPS) signal coming out of the GPS, the PPS signal is attached to an interrupt, the attached ISR sets the MCU timer to zero and records the time stamp from the GPS. When an event occurs the remote nodes send a message that such and such happened at so many clock cycles from the time stamp reported.
When the message gets to the base it can store it and time correlate to all other messages. This should give me time resolution to about a microsecond with no problems.
wade