engine vibrates 4th/5th gear when driving uphill

I have a 1997 Madza B2300 truck shortbed that runs very rough in forth and fifth gear when going up hills. As long as the truck is driven on "level" roads, the car runs fine. It has been in a couple of mechanic shops and I've did/had done some preventive maintenance but the problem continues.
Changed oil to a lighter weight per owner's manual. This did improve the engine's performance.
Changed shark plugs/wires. Problem continued.
Changed timing belt. Problem continued.
Checked Mass Air Flow Sensor/okay. Problem continued.
Ford deslership checked the PCM proportional/okay. Problem continued.
Checked catalyist converter/okay. Problem continued.