The front axle changed from a solid or straight Dana44 in the 79 4WD to a TTB or Twin Traction Beam Dana44 in 1980-96 4WD F-150's. While the axle swap (SAS) is popular in the 80-96 models, it's not a simple bolt-on and requires some fabrication. The frame was also changed from 79 to 80, along with body panel style and mounting points.