What can cause the van to shake when I accelerate between 25 to 45?

misfire or torque converter , based on limited info.

Torque converter... Ford has a recall on them and will fix it for free BUT if can find ANY possible way to either do it yourself or pay someone to replace it with aftermarket parts and NOT let them fix it you will be exponentially better off. Take it from someone who been there done that and gotten WAY too many t-shirts to go with their freestar