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.
Actually, for S2 XBees,
Special definitions for DH and DL include 0x000000000000FFFF (broadcast) and
0x0000000000000000 (coordinator).
So the end with the router can have DH=DL=0 and it should work fine, I actually do this a lot. However, since in this case the comm is one-way, it really doesn't matter (assuming the end with the pot has the coordinator). Put the coordinator on the end with the servo, and then both XBees could be left at the default setting of DH=DL=0.
Edit: Note simplest XBee S2 configuration for one-way transparent operation consists of simply loading coordinator firmware on one XBee and Router firmware on another, and then using the Router as the sending end. Assuming no other networks are in operation, this should be all that is required. If there are other XBee networks, then set the PAN ID for both units to some value different from the other network(s). I always like to set a unique PAN ID anyway, but the default value of zero should work fine as this causes the coordinator to select a random PAN ID and the router to join any PAN ID available.