I've heard that someone actually makes a router that works with Verizon and Sprint connection cards, which would be your best choice, if you absolutely can't get cable or DSL. You can also try to use internet connection sharing, but it doesn't work very well, in my experience. DNS doesn't forward properly and stuff. You'll have to manually setup the XBOX's IP settings. The ICS wizard should give your laptop the address 192.168.0.1 (WTF?), so you'll need to set the XBOX to
IP: 192.168.0.100
Mask: 255.255.255.0
Default gateway: 192.168.0.1
DNS1: 209.120.229.62 (that's Zelaron)
DNS2: 192.168.0.1 (if Zelaron is down, it's possible your DNS forwarding will be working ATM...)