I decided to try some good old fashioned WD-40 tonight after work, and at least for now it seems to have fixed the problem. The creeking/vibration seemed to be coming from the area where the bracket contacts the body bolt, so I just doused that area and called it a night.

We'll see how long it lasts. If WD-40 isn't the long-term solution, I'll try silicone spray next and then work up to RTV. Thanks again for the suggestions Tom!