What's wrong with my van?

I was just given a van by a sweet couple who knew I was struggling as a single mom. Its a 2005 ford freestar. The problem is when I get on the highway and get going between 50-65 it shakes for a few seconds and then stops. If I let off the gas it stops, and of I push the gas harder it stops. I have floored it and it shifted smoothly through all the gears and did not shake. I just wanted to see if anyone had any suggestions as to what the problem could be. I'm kinda nervous to drive it far in case something happens.

http://autorepair.about.com/