I changed my u-joints over the weekend, and the book at the parts store called for size 354 for an F-150. Well, after I got my driveline out, I saw that they were way too small, so back to the parts store I go. The ones I finally got (that matched up to the ones I took out), were listed in the book for an F-250, a size 331. Is this unusual? Is it because I have a steel driveshaft instead of aluminum? What gives?