JK,
You can create a very basic packet sniffer on your router by creating an access list consisting of your source and destination hosts/networks and protocols in use and then run a debug against the access list. If you are using CEF on any of the transit interfaces you will need to turn it off to capture the packets ("no ip route cache cef" i think)
e.g from a telnet session
# access-list 187 permit host 1.1.1.1 host 2.2.2.2
# logging terminal debugging
# terminal monitor
# debug ip packet detailed 187
HTH
Paddy