From my previous experience, whenever there is a squeaking noise, especially going over bumps. Its usually some bushings. Fords always had big issues with these. Most older Explorers and Rangers squeak all the time. Its pretty annoy. Could just replace the bushings with polyurethane ones. They tend to be pretty cheap. And usualy not too hard to do either. Some companies like Energy Suspension on Summit sell whole kits for like $50-$150.