Keselowski began competing in the Truck Series full-time in 2005 with backing from SUBcrews.com and Samson Stone. He opened the season with a seventh-place finish in the Florida Dodge Dealers 250 at Daytona, his only top-ten finish of the year. He would end up finishing 21st in points. He ran the first two races of 2006 for K-Automotive before a lack of sponsorship caused the team to temporarily suspend operations.
Keselowski then drove the No. 02 Chevrolet at Kentucky and Memphis, filling in for the injured Kelly Sutton, and drove the No. 63 Ford for MB Motorsports at Bristol and in the season finale, the Ford 200 at Homestead.
In 1918, the Samson Tractor division of General Motors was formed from the merger of Samson Tractor of California and the Janesville Machine Company, a farm implement manufacturer. A Samson Tractor Division plant employing 3,000 workers was opened in Janesville in 1919, with the first tractor produced on May 1, 1919. A farm depression that started in 1920 forced Samson to curtail production and lay off more than 1,000 workers.
Samson eventually went bankrupt. Although General Motors wanted to abandon the Janesville plant, its general manager, Joseph A. Craig, convinced the company to stay, and in 1920 GM transferred its truck production from Flint, Michigan to the Janesville plant.
Keselowski began competing in the Busch Series full-time in 2007. He drove the No. 23 Chevrolet Monte Carlo SS for Keith Coleman Racing, until the team suspended operations in July. Earlier in June, during the 2007 Truck Series season, Keselowski was tabbed by Germain Racing to replace Ted Musgrave in the No.
9 Team ASE Toyota Tundra for the running of the O'Reilly 200 at Memphis after Musgrave was suspended for an in-race scuffle with another driver at the Milwaukee Mile. Keselowski won his first career pole for the race, and led 62 laps, but got turned around by Travis Kvapil as they fought for the lead with ten laps to go.
Keselowski wound up finishing 16th. Shortly afterwards, Keselowski was called by car owner Dale Earnhardt, Jr. to drive the No. 88 United States Navy Chevrolet for JR Motorsports for the rest of the Busch Series season.
During the Camping World 300 at California Speedway, Keselowski was involved in an accident involving A. J. Allmendinger and J. J. Yeley. Keselowski was able to climb out of his damaged car, but complained of foot pain. He would later be treated and released from Loma Linda University Medical Center, and was cleared to race at Richmond the following weekend. Keselowski would close the 2007 season out with five top-ten finishes and a 25th place finish in points.