Who bought out Ford semi trucks?

That division of Ford was bought out by Daimler A.G. in 1997. Daimler A.G. is the parent company of Mercedes-Benz, Freightliner, Western Star, Thomas Built Buses, and others. They were also the owners of the Mopar group (Dodge, Chrysler, Jeep) until they sold that to Cerberus Capital Investments in 2007.

Daimler A.G. produced the Ford trucks under the Sterling name from 1997 to 2009.