I put LEDs in my 92 D250 and the Imperial. They dim just like the original lights.
My LEDs came from superbrightleds.com.

Here's a couple shots to show the range.






It's a very linear progression. I very rarely keep the gauge lights all the way up. It's pretty bright.
I will give you a word of warning. Don't use the brightest LEDs for your indicator lights. I did that in my truck. When I hit the turn signal the interior of the truck flashes green. It's way too bright. Using the hazard lights is almost blinding at night.



Use green LEDs for green lights and other colors for other color lenses. If you put a white LED behind a green lens like those turn signal indicators the LED color will blow through the tinted lens. A white LED behind the green lens would be a white light with a slight green tint. White LEDs in tail lights will shine a very pale pink.
Always match the LED color with the lens color.
My truck's gauge lights are cool blue LEDs with the original blue bulb covers.

I put ultraviolet LEDs in the dash flood lights in the Imperial. Since the gauges are lit from the dash pad and not from behind the gauges the UV LEDs reduce glare while really making the gauge markings and needles pop nicely.