1994 Ford Explorer Shaking, poor gas mileage, and bad accel

I have a '94 Ford Explorer that has recently started shaking pretty badly on the middle of the right hand side. It's now getting really bad gas mileage, too. It never seems to warm up now, and in the last few days when I drive it on the highway it takes forever to get to 60, jumps to 65, and can't even get to 70mph. What is wrong with my car, and how much will it cost to fix it? Thank you!