It may not be the strut itself, but rather the strut top mount. Seems this is a common failure point on these Explorers. I think I had the same problem as you... it sounded like a dull knock when crawling at low speed in a parking lot, but no issues with ride quality. Fortunately for me, the dealer replaced front struts, mounts, and links on both sides under extended warranty at around 40k miles when I was having this issue.