If BIG-IP is synced to your NTP servers, why not just use one of the self-IP's as the server and eliminate the extra call? My XP machine sync's just fine to my self IP:
[root@ltm-dc01:Active] config ntpdc -p
remote local st poll reach delay offset disp
=======================================================================
=ntp-1.gw.uiuc.e 10.10.10.10 2 256 377 0.02010 -0.014216 0.00352
*time.missouri.e 10.10.10.10 2 256 377 0.02060 -0.012398 0.00449
As verified in tcpdump:
[root@ltm-dc01:Active] config tcpdump -ni 0.0 udp port 123
tcpdump: listening on 0.0
14:12:10.705663 802.1Q vlan10 P0 10.10.10.240.ntp > 10.10.10.10.ntp: v3 sym_act strat 0 poll 10 prec -6
14:12:10.706130 802.1Q vlan10 P0 10.10.10.10.ntp > 10.10.10.240.ntp: v3 sym_pas strat 3 poll 10 prec -16 (DF) [tos 0x10]