problems with '98 Expedition?

Here's the deal. If I'm lucky, my truck will start out driving just fine, but when I hit about 40 mph, it almost feels like it is shifting into too high a gear. I have my foot on the floor, and the RPM's are going as high as 4500, but i lose speed. It will normally stay this way for a while afterwards, even after I shut the car off. If it's in this "mood" it does the same thing in reverse, which leads me to believe that it's not the transmission (since there's only one R gear). I'm wondering if it may not have something to do with the gas? Maybe it's not being fed to the engine properly?. I'm lucky if I can get the thing up to 50mph and maintain it. Even if I get up that fast and turn the cruise control on, it does the same thing.

And yeah, I know, take it to a mechanic. But my husband is currently deployed and I hate to just walk in and have no clue, and run the risk of getting ripped off.

Any suggestions would be greatly appreciated.