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Yes, people have. Most I've seen have either removed them entirely or gone with a retractable running board. That doesn't sound like what you're looking for. I'm not aware of any designed specifically to keep mud off your truck, since that's not exactly what they are built for. There is a thread on here where someone plugged the holes in the board rather than replacing it. This was done so that dog paws don't get caught in them, but might serve your purpose as well.Also, I'd look into getting some mud flaps if you haven't already. That will likely help with keeping mud/rocks off the truck than the running board.
Yes, people have. Most I've seen have either removed them entirely or gone with a retractable running board. That doesn't sound like what you're looking for. I'm not aware of any designed specifically to keep mud off your truck, since that's not exactly what they are built for.
There is a thread on here where someone plugged the holes in the board rather than replacing it. This was done so that dog paws don't get caught in them, but might serve your purpose as well.
Also, I'd look into getting some mud flaps if you haven't already. That will likely help with keeping mud/rocks off the truck than the running board.