Dust bathing is really good for your chickens. It dislodges the parasites and also helps get dirt and oils off of them. Make sure your chickens have an area to dust bath and if they don’t, put a big pan filled with sand in their pen. I have a covered area with wood ashes for them to use. The re-infestation from wild birds is a constant problem. 
Northern fowl mites are obligate bloodsucking parasites that can complete their life cycle in 5–12 days. Off the host, mites may live up to 4 weeks, depending on temperature and relative humidity.
Dust boxes with sand and acaricidal materials, such as inert dusts (eg, diatomaceous earth) or sulfur dust, are very effective for bird self-treatment of mites. Once established it will be a long process. Removal of the birds from the infected area is necessary. The coop should be thoroughly cleaned, perches and all cracks and all boxes. All bedding must be removed completely. The area can then be pressure washed or  soaked with a Lysol spray which may have to be repeated. I wish I had a single easy recipe, but these are really bad news.

Hi there, Thank you so much!  I do have a few follow up questions:

The mites seriously weaken the chickens. We just dealt with that problem here, completely clearing the old pen etc. We do have a separate coop area for new chicks etc. so that does help. After cleanup, the spray also helps. As to the limping, there are a number of possibilities including injury, disease, scaly mites (which specialize in burrowing into the leg scales, raising them up). A common one is Bumblefoot Staphylococcus (Staph) infection that usually starts on the bottom of a chicken’s foot. Bumblefoot is easily recognized by a small, dark scab typically found in the center pad of your chicken’s foot. If not treated, it can spread throughout its foot and into its leg. This disease is transmittable to humans, so wear protective gloves. It could, of course, also be a sprain or thorn.