It sounds like one of your many hip flexor muscles, and if it is not pulled too bad, you should be OK with walking and anything else that doesn't hurt. Going fast on any surface would not be a good idea because it requires your legs to swing farther and faster, which can be hard on these muscles.

Did you catch your foot on something while getting into the truck? Normally, it would be hard to pull this muscle by running, unless it is really fast, or on challenging terrain. You didn't mention an upcoming race or training program, so hopefully this injury came along at a convenient time, and you can save yourself a few miles and get some other things done while it heals. If you are kind to the injury, it should be back to normal within a couple weeks.

In the meantime, I would avoid speed work and any stretches that extend the leg too far to the rear. Familiarize yourself with the pelvic muscle group, and what muscles perform which actions, so you know where to focus any massage. Here's an interactive animation of the relevant muscles for study...

http://www.getbodysmart.com/ap/muscularsystem/thighmuscles/anteriormuscles/menu/menu.html

All of the adductors, except for the Gracilis, are hip flexors, too. This helps to explain why the pain kicks in during seemingly unrelated movements. Running calls upon a larger group of muscles for flexion than other activities, in order to spread the load and rest some fibers while others are active.

The adductor/flexor most likely (as described) is the Pectineus, high and shallow on the inner thigh, right on the surface of that web from the groin. Refers pain to the hip socket. One way to distinguish it from the others is that the fibers are heavily striated, like corduroy when felt through the skin. The fibers move at roughly a 45 degree angle from groin to upper inner thigh. I would not ice this area, but the heat may help.

Try getting a thumb on it to check for sensitivity. It is not normally a sensitive muscle, despite its location. Hard to let anyone else touch, although I did work on a triathlete's in public once by her request. She said it was a recurring problem after she strained it and continued training. Not much area to work because it is a very short muscle, shown in the link I posted above.