Your best bet is probably, like mentioned above, to assign a different port and do port-based dnat (port forwarding) to your internal servers based on their ports.
Example:
Map 3389 to your Internal server (192.168.0.5)
3390 to another machine (192.168.0.6)
3391 to another machine … etc..
Then, using MSTSC, you can specify an alternate port by using the WAN_IP:port syntax (64.34.153.10:3390)
But it would be considered a better practice to open these ports through a VPN (PPTP works well) or at the very least, limit access to a given source IP address.
Guillaume Bélanger
http://www.exosource.com