1991 F150 transmission

I am getting ready to take my truck to a transmission repair shop. Before I go I thought it would be nice to at least know what the problem might be. As I am driving (this has been happening on the freeway as I am just crusing along) the transmission seems to shift out of gear. The engine races and once I then slow down the transmission seems to shift into gear and after doing this a few times seems to work. This does not happen every time I drive it. The truck is not driven alot and only has 65,000 miles on it. Am I in for an expensive repair?