Attached is a diagram illustrating my situation. I need some nodes in the 172.16.5.x /24 network to coexist at two seperate remote locations. Currently everything resides at Router B. Router B is the default gateway for that network (172.16.5.1). We run EIGRP and have multiple VLANs (hence the subinterfaces) The DHCP server for these nodes is attached to Router A. My question is, how can I put nodes on Router C in the 172.16.5.x network, with the default gateway on a different router? What IP should I give to the subinterface on Router C (or should I give it an IP at all?)
what you need here is a L2 transport service over IP.
if the WAN links were serial links you could use IRB with bridge-group commands on dedicated lan interfaces and on the serial link between RB and RC.
However, you would like to use LAN subinterfaces and the backbone links appear to be a LAN interface with IP subnet 172.16.50.0/24.
I would use L2TPv3 to setup a L2 point-to-point transport between RB and RC.
A link to L2TPv3 config
this feature is supported on ISR routers like 2811 for example.
This is reasonable if traffic volume is not very high.
the only problem is the default-gateway : for that you should need to define a subif with same 802.1Q tag = same vlan number on a different physical interface connected to another port of the lan switch.
This because the l2tpv3 vlan subif has no ip address at all, it is providing a L2 service.
You need a second LAN interface in one of the two routers, it could be better if using also a second lan subif on the second router to provide L3 redundancy
Hope to help
L2TPv3 would allow you to extend the L2 vlan across a routed WAN.
Note that not all routers/IOS versions support L2TPv3 so you need to check with Cisco Feature Navigator.