The brush that comes with the Mr. Clean Pro series (car wash kit) works great on that area. It also works good between the tires and mud guards. I bought the kit didn't like it, they (Sams Wholesale Club) said I could keep the brush.
Couldn't find a picture of the brush on the internet, it's like a "loop shaped brush with bottle brush bristles and a slightly curved handle"
I saw a similar brush near the cleaning products in an auto parts store.
Thanks 06RTLNAVBlue and Baja for your replies. Now that you mention it, I did see a brush similar to your description at the auto parts place. Baja, I can't get my hand between the spokes, let alone with a sponge or mitt. It almost looks like it may be easier to lay under the truck and clean this part from the inside. Then, I would have to move the truck so each wheel turns 1/2 revolution and repeat. The brush, if not abrasive may allow me to go between the spokes.
You may want to consider looking in the kitchen gadget section of a well stocked kitchen supply store.
There are many sizes/shapes of "sponges on handles" that may work for this application. Some hold your cleaning solution in the handle. Some have replaceable sponges and cleaning pads with different soft abrasion levels.
I use a soft abrasive tapered flexible sponge on a handle for the curved edges of my wheel spokes with good results.
I use a pressure washer (1300 psi). I let it soak with wheel cleaning solution, then blast it clean. I use the pressure washer to clean the rest of the truck too. I use the dispenser to apply the soap first at low pressure, rinse with high pressure, dry with a microfiber cloth.