I am so glad I wrote in because that helped me to know what to look for! Yesterday, my husband and I left the house for a few hours, and when we got home, Keekee was getting up from a nap and my husband witnessed the behavior too. I didn't get a chance then to look at his underside...but I did just a few minutes ago. This morning my husband let him outside. No behavior shown. I let him in a bit later after playing fetch some...no behavior shown. Keekee evidently took a nap when I went out to do some errands. I got home and once I walked in he started this again! He seems to be doing it more and more! I pulled back his hind leg a bit to see if I could view his penis unsheathed, and sure enough, it was sticking out as he continued to move his hips. Oh my.
I feel so ignorant, but I genuinely worried that he was doing this to "wake up" his hind end in an effort to stand...as if he was having some difficulty of some sort.
Thanks to all that responded. I think this is a simple case of Keekee becoming a grown up dog, and displaying behaviors matched to the growing process. Since I saw the underside of him, I thank goodness that's all that this is.