I purchased some center caps (rear) a couple years back from this company. Price for two is $55. Might try them. You have to line up and play with the lug nuts a bit to make them fit.But mine have not come off - they do not use the tabs as the Ford caps do.
http://www.suburbanwheelcover.com/cgi-bin/WebObjects/swc-prod.woa/wo/12.0.0.7.0.3.0.5.15.3.0?45,14
Dan
http://www.suburbanwheelcover.com/cgi-bin/WebObjects/swc-prod.woa/wo/12.0.0.7.0.3.0.5.15.3.0?45,14
Dan