1992 Mazda B2200 engine hesitates and makes truck shake

1992 Mazda B2200 mileage: 142,000. It has been missing (I think, the engine hesitates, and actually makes the truck shake) in forth and fifth gears, especially in the morning and going up hill. If I let off of the gas, or down shift, it helps but still struggles to get up hills. It is only really noticeable on the freeway. I just replaced oil and filter, air filter, plugs, wires, dist. Cap and rotor button. I am still having this problem. It doesn't seem to happen in gears one through three (or is not that noticeable). Any ideas?