check for vacuum leak or dirty fuel filter
clutch switch is bad
Your engine produces torque(turning power) and revs, and these are related. You can't get more than a certain amount of torque out of the engine at a given number of revs. If you hit a slope in the wrong gear/at the wrong speed, then forcing the car forward will require more torque than the engine can deliver, and it will stall. It's very much like trying to ride a bike uphill in the wrong gear.
If your car stalls when you put it in gear, it could indicate a problem with the transmission, engine, or fuel system. Common issues include a faulty clutch, low transmission fluid, or a malfunctioning fuel pump. Additionally, it may be related to a vacuum leak or issues with the idle air control valve. It's advisable to have a mechanic diagnose the problem to ensure safe and effective repairs.
engine stalls at low speed
Probably a clutch, but might be a problem in the trany.
When a 1987 Ford F-150 stalls upon being put in gear, it may indicate issues with the transmission, such as low fluid levels, a failing torque converter, or a problem with the clutch (if it's a manual transmission). Additionally, it could be related to the engine, such as a vacuum leak, faulty sensors, or fuel delivery problems. Checking the transmission fluid level and condition, along with performing a diagnostic on the engine, can help pinpoint the issue.
Not sure when you say low speed. Not knowing what your driveline ratio is I will just use some basic numbers .If 20 MPH is your speed and you were in first gear your engine RPM's would be 3000, if you were in second gear at 20MPH your engine RPM's would be 2500 , if you were in third gear at 20MPH your engine RPM's would be 2000. A difference of 1000 RPM's from low to high gear. Lower engine RPM's less fuel used.
In a low gear is better than any other on ice. low gear is the best choice it allows you to slowdown with the engine and have more control.
Low RPM. Gear is too high for travel speed
you are going too slow for that gear your engine has very little torque at low rpm
maybe your idle to low, if it continues, I have the fluid injection relay switch replaced.
Where is the best place to park your ATV after shutting off the engine