Hello and good afternoon. The Vehicle speed sensor is located on the left rear section of the trans, near the tail section. Here is a picture. Note the picture is with the transmission positioned upside down, but it gives you an idea what you are looking for.

Your JA Expert,

Ivan

If the speedometer is reading properly, I don't think so. If the speedo is malfunctioning too, then definitely. If it's slipping, its very probable that you have internal wear causing your issues. A lack of shifting could go towards a shift solenoid, but slipping into neutral is usually internal wear like worn clutch packs or the loss of hydraulic pressure.

The problem is most like the result of a slightly if not large difference in gear ratios. If the ratios are not exactly the same or tires being exactly the same in total circumference the binding sensation of the tires rolling at different rates will cause unsafe if not catastrophic results. The difference in axles may be yielding different ratios. That is where I would focus. If they are OK and it's only on bumps, you may need the aid of a steering stabilizer added to help control those osculations. Check for any play in ball joints or steering components as well. Excess play anywhere, will magnify with the lift and larger tires.

If the tie rods rotate, that ok, you don't want end play. Try this, with the truck on the ground, turn the steering wheel and have some look at the track arm. Look for play where it connects to the frame. Look for play. Same where it connects to the differential. You won't see this play unless the weight is on the wheels.

You may have to find a dual stabilizer kit. Also, look at the toe settings. If the wheels are toed in, they can cause some of that behavior, once the suspension is upset by the bump. Even 0 toe could cause a wander when the bump occurs and that magnifies into this behavior. If all is tight, it could be a matter of geometry from this standpoint.