Can you downshift a ford 350 diesel to slow it down if you don't have an exhaust brake?

Can you downshift a ford 350 diesel to slow it down if you don't have an exhaust brake? - 1

Yes

Sure.

The lower gear will slow the truck down even if the exhaust doesn't.

Hi so what is an exhaust brake never ever heard about on of those in 49 years of motoring.

People have been downshifting as a means of slowing down since the invention of the motor vehicle. Less so in modern days as braking systems have improved.
Most truck drivers will still downshift as a means of controlling the speed of their vehicle.
An exhaust brake is simply an additional means of braking using the engine compression as opposed to the brakes and can be used in conjunction with gears and brakes.

Every vehicle with a manual transmission will slow down when you down shift and let off the gas pedal.

Though the answer is yes. Be aware of causing over revving? This is where transmission drives engine speed to to weight and speed of car/truck. Causing excess engine speeds. Over 4500 rpm for a diesel for example often has valve bounce /lash occurring that will have valves impacting on pistons. This can cause massive engine damage. However normal added engine braking is Ok.