This follows three of the best answers I've ever read on this site, but I feel obligated to do a little more than pile on. There's more to know.
Many joints in the body, sometimes referred to as "sutures," are not thought of as joints because they are not meant to move much, more as "safety valves" to prevent bone breakage, or vestigial from fetal development. Your pelvis, like your skull, is knit together from such bones, and surely has to compensate for the lack of movement in your fused spine as Damien notes. This would be doubly true when running on uneven terrain, as you recounted above.
Movement within the pelvis is anatomically limited by several ligaments in addition to the aforementioned semi-rigid joints. However, most athletes, and a lot of professionals, spend far too much time obsessing about joints, rather than the substantial musculature that controls and stabilizes them. Most of our training is focused on strengthening these muscles - it's the main reason we train - but every now and then, that strength comes back to bite us like an overtrained dog. Much of the pain athletes experience can be attributed to overworked muscles, whether or not the resulting pain seems like it is coming from the area of the body that seems to hurt.
After spinal fusion, you could expect the orchestration of more than one group of muscles to limit the movement of the bony plates of your pelvis, and you could expect them to tire. You could also anticipate that they might become knotted, might swell, might lose function, and might exhibit pain syndromes that manifest in locations described as "butt" pain. However, it can be more complicated than that.
Numerous are the various core muscles involved with gluing together the upper and lower parts of the body, and you have mentioned some of them. Familiar to many, are the symptoms of false sciatica due to irritation of the sciatic nerve by the piriformis muscle. Other knotted or swollen muscles line the path of the sciatic nerve branches, and I am most concerned in your case with the health of the gluteal muscles, because of how important they are when descending hills, as you recounted above.
Branches of the sciatic nerve run between the gluteus minimus and medius specifically, and pressure between these muscles can affect the nerve in the way you describe. While exercise and strengthening of these muscle groups is the province of trainers and PTs, the popular characterization of these muscles as "weak" or atrophied from too much sitting or other causes is not enough to know. Weakness is one condition that can affect operation of these muscles, but their tone, shape, and operation can be altered in other ways, the impact of which of which can be felt down the leg, exacerbated by repeated use in downhill running.
What to do with core muscle mutiny? Outside of those in the vital organs, the muscles in your core that hold your pelvis together and to everything else, are pretty tough to access. While tools like the tennis ball Jim mentions can tell a lot and help a lot, some of these important fibers are completely out of reach. Your desire to stretch is born of this unreachability, but as Damien suggests, can be fruitless or counterproductive. It may take some time for symptoms to die down on their own, with a lot of care thrown in for good measure.
By all means trust the professionals, but verify what they tell you. Each professional is shaped by training and personal experience into a unique combination of knowledge, intuition, and personal prejudices. Recognize that the internet chatter you have consulted is a two-edged sword, where junk information shares the spotlight with clear insights not muted by professional obligation to close ranks. If you feel you are bright enough to tell the difference between what is and isn't helpful information, you probably are, but you must have the patience for the data to settle into place. Just allow for the fact that everyone, including the professionals, gets it wrong every now and then. The difference is that it is easier to hold your provider's feet to the fire in the professional relationship, and the information you have collected from other sources, including the internet, will help you do just that.
In my view, the web has quickened the dissolution of long-held beliefs and strategies within the healthcare establishment, and has accelerated beneficial changes. Yes, there are unfortunate and counterproductive trends spawned by internet overexposure, but another way it fails is as a repetitive mouthpiece for the status-quo, plenty of which can be seen out there. Some facts are timeless, or apparently so, while others fade as a mirage in the light of ongoing research. If your docs can keep pace with all that, more power to them, but without the drag of professional culture, you certainly can. Good luck with your injury and your continued running, and please keep in touch with the rest of us, so many can learn from your experience.