PaulS:
Does DH and DL for the XBee on the potentiometer end equal SH and SL for the XBee on the servo end?
And are they non-zero on both ends? 0 is the broadcast address, and broadcast communications are lower priority than directed communications.
Yes, and broadcast also causes more network overhead.
I had the parts handy, so I tossed it together, worked fine for me with the code I posted above. Even with the 100ms delay response seems immediate.