Cisco WAN :: 1841 - Routing Between Ethernet Ports

I have a Cisco 1841 that has an ADSL (ATM) card installed.  It was previously used with an ADSL line to provide NAT routing for an office.  Now I want to use it witha cable modem, which would mean abandoning the ADSL interface and instead routing between the two Ethernet ports.Between the Cisco and the LAN is a Linux transparent proxy.  It provides routing between 192.168.1.0 (LAN) and 192.168.2.111 (Cisco LAN interface).  The network looks like this:
 
Cable Modem(174.76.21.1)(gateway) --- (174.76.21.10)(WAN) Cisco 1841 (LAN)(192.168.2.111) --- (192.168.2.11) Linux Proxy (192.168.1.10) --- (192.168.1.0) LAN
 
For testing and diagnostics, I've connected a laptop to each FastEthernet port on the router.  One laptop is configured with the IP 174.76.21.1 to simulate the cable modem gateway and the other laptop has the IP 192.168.2.11 to simulate the Linux proxy.  From those systems I've performed the following diagnostics with the following results:
 
From 192.168.2.11
Ping 192.168.2.111 - OK
Ping 174.76.21.15 - OK
Ping 174.76.21.1 - Timed out
 From 174.76.21.1
Ping 174.76.21.15 - OK
NAT translation to LAN IP's failing
 
There are some vestiges of the ADSL configuration, but I've cleaned most of it out and shut down the ATM interfaces.  Here's my config:
 
!
version 12.4
service timestamps debug datetime msec
service timestamps log datetime msec
no service password-encryption

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