2000 Ford Ranger runs great when parked, lol

I have had this truck for 4 years and never really thought much of the "problem" untill now that gas is $4.00 a gallon! The motor seems to run great all of the time, but when I need to accelerate. Weather it be from a lite or to merge on the freeway, it has no power or I would almost say a trans slip but there is no noise or grinding. The rpms seem to be right where the need to be, im just not accelerating as fast as it should be. And I seem to be geeting about 16 mph highway with it right now. I have already changed the: feul filter, plugs, wires, air filter, oil, and cleaned the MAF, and throttle body. Any suggestions?