The oils in your skin is what traps the moisture in and keeps the skin cells alive and healthy. Healthy, intact skin is your body's primary defense against infection. Having dry butt skin is akin to pitching a tent next to a dumpster.
Prolonged showers, particularly with very hot water, and harsh soaps, will strip the oils in your skin, and will dry it out. If you get out of the shower and your skin feels all clean and tight, it's dried out. Antimicrobial soaps, all of which have no proven medical or sanitary value, will also dry out your skin.
Most soaps, and laundry detergents, that contain perfumes can also dry out your skin. Mechanical tools, like harsh sponges, sandpaper, Lou Fahs all will damage the skin and strip the oils and dry it out. Sandblasting your skin only temporarily removes the symptoms (the skin flakes), but doesn't treat the underlying problem. In fact, it makes it worse.
Don't moisturize your skin when it's dry. It's, uhm, a little late at that point. Despite what the people who make moisturizer will tell you in order to get you to buy their moisturizer, a moisturizer simply replaces the lost oils and will trap moisture into the skin. The time to apply moisturizer is when your skin is moist and damp, but not wet, like after you've towel dried after a shower. The moisturizer should be thick, very thick. A dollop in the palm of the hand should not run or drip when the palm is turned over. For dry skin, thicker is better. Just don't apply it thick. Apply a thin, even layer, then pat away the excess with a towel.
If your skin is otherwise relatively healthy, but only your butt cheeks have the problem, I'd take a close, long hard look at your butt cheeks and, oh, uhm, sorry, lost my train of thought there for a second. I'd try and figure out what your butt cheeks are having to deal with, that the rest of your skin doesn't have to deal with. Namely, the biker-boy panties.
As in, what are they made of, and what are you washing them in? Might be the fabric, the laundry detergent, or the combination of both. If the panties are the kind that are designed to let your skin breathe, to let air in and let moisture out, there ya go. The incoming air is drying your butt cheeks out, and the high tech bloomers are letting the moisture escape, resulting in a little more dust added to the planet every time you drop yer drawers.
Also, take a look at the driver's seat cushion, or lack thereof. If there's a cushion there, wash it or get a new one, and if there isn't one, get one.
Butt, you have to be honest with yourself. Do you really and truly want to get rid of the dandruff? For if you do, you can kiss that baby oil enema thang goodbye. Whew-weee, thaaat's a tough call.
Slow and steady, even in expediting, wins the race - Aesop