If you want a connection while driving, there are a few options:
- Pay for a connection card through Sprint (or anyone, but they're all the same price per month, and you already have them)
- Add "phone as a modem" to your phone now (while connected, phone calls will probably route to voicemail only)
- Add a line to your existing account with "phone as a modem" on it so you don't miss any calls
- Boost mobile trick (check the sticky)
- Wifi Card, try to get signal as you drive.
The most reliable and secure would be either the phone as a modem or the connection card since you're receiving a direct connection from the ISP, rather than routed through a wifi hotspot. It's a little more expensive per month, but that is something you have to evaluate. Is it worth that much every month to pretty much guarantee a secure connection?