I noticed that it looks like you have the same type of round tractor lights on the back of your truck that I do. Are you able to see the ground ok and the salt being spread?? I found that the tractor lights that I have do not light up the ground enough and I can barely see the salt being spread. I just added a second set of lights to mine but have yet to spread so I don't know if they made enough of a difference and may be looking for something brighter.
Gordyo, go to your auto prts store and order #4466 replacement bulbs. I've yet to find anyone who stocks them, so they'll be special order, sometimes even backordered. They are a halogen version of the 4" light you have, and have a tighter fluted lens. They throw about twice the light that the regular bulbs do, once I discovered them, I haven't used anything else.
They are Whelen halogen rotators. I didn't want them mounted above the top because I thought they might be damaged when loading. We have a Whelen strobe bar on the roof. The tractor lights are more for general illumination when backing. I also added a light to shine on the drivers side of the spreader chute to show where the salt was spreading. You can see in this picture the light shining on the ground. I agree a brighter bulb would help. These tractor lights were only $7.95 at NAPA.
The one light on the side of the chute is adjusted so that it points to the outside of the arc of the spread salt. It's handy at night to just line up the light with the edge of the roadway or lot, and be reasonably assured that the salt is being spread in the right area.