The 1997 Ford F150 base model came with a 4.2-liter six-cylinder engine and a five-speed manual transmission. Drivability problems like bucking or jerking can be a hard thing to diagnose because the source could be from the engine, transmission or rear end. The best thing to do is ride down the road when you can duplicate the problem, roll the windows down and listen for any noise that is being made when your truck begins to buck and jerk. With some careful listening you may be able to get a rough area of the source if it is a mechanical problem.
The Rear End
You need to consider the last time your rear axle and differential was serviced. Your rear end may have what is known as limited slip, which means the two axles connecting the rear wheels to the differential can spin at different speeds. This is very helpful for turning around corners as the inside tire naturally travels slower than the outside tire when cornering. To control the action of a limited slip differential your rear end has two multi-disc clutch packs If these clutches have become worn or broken you may exhibit a bucking or jerking sensation. Even if you don't have a limited slip rear end you may also want to consider the last time the fluid was checked or changed; low or extremely worn fluid can cause the differential to jump due to lack of lubrication; possibly causing damage to the teeth inside the differential. You can review your original paper work or do a VIN search to determine exactly what your vehicle was equipped with from the factory.
Take into consideration the fuel system and how well it has been maintained over the years. Being a 97 model year, your truck has become well aged and the fuel system may require some attention. As a rule of thumb if you cant remember the last time you replaced your fuel filter than you should probably just replace it anyway; a clogged filter can cause serious drivability issues including a bucking and jerking as your engine is starved for petrol. The best way to test fuel system operation is to check the fuel pressure at the fuel rail with a fuel pressure gauge. The test port is on the passenger side of the fuel rail just before the fuel injectors. Install the pressure gauge to the service port; with the key on and the engine off, fuel pressure should be between 30 and 45 psi. Any pressure below this threshold links back to a failing fuel pump.
The ignition timing for your engine is electronically controlled by the PCM based on information from the camshaft position sensor and the crankshaft position sensor. A failing coil pack will not distribute the correct spark, if at all causing and intermittent buck and jerking as one or more of your cylinders misfire. Check your spark plugs and ignition wires first. Your spark plugs should be gapped at .054-inches and should be tightened to 12 to 27 foot-pounds. Check your wiring harness connectors at the coil pack, the crank sensor and the camshaft sensor. The crank sensor is located behind the harmonic balancer on the passenger side of the engine block and the cam sensor is located on the front of the engine at the timing cover. You also want to check the main connector at the PCM for any corrosion and clean or replace as necessary; it is located behind the passenger side kick panel and is the heart that runs your engine and transmission functions.
Throttle Position Sensor
The throttle position sensor is a problem well known to a lot of Ford truck owners. The throttle position sensor is a variable resistor that is mechanically connected to the butterfly valve in the throttle body. A reference signal is sent from the PCM to the throttle position sensor. The variable resistor changes the voltage sent back to the PCM based on the position of the throttle. A dead spot will develop at the most commonly used throttle position point causing the fuel to be cut off from the injectors when the dead spot is crossed. This quick cut off will cause the engine, along with the truck to buck and jerk as it is starved for fuel. If your most commonly traveled speed is 35 MPH and that is where you exhibit the bucking and jerking, it is very likely the throttle position sensor is at fault and should be replaced. It is mounted on the throttle body opposite of the throttle linkage.