Neither Rain, Nor Sleet . . . Nor Dinosaurs: The U.S. Postal Service and MCA Home Video are the latest twosome to join forces for a cross promotion. Their efforts involve the videocassette release of the George Lucas-Steven Spielberg animated dinosaur adventure film, "The Land Before Time," and the postal service's new commemorative dinosaur stamps. When the video hits the market on Sept. 14, each tape will have an insert promoting the new stamps, which will be available Oct. 1. Post office lobbies in turn will have posters promoting a dinosaur T-shirt offer inside the "Land Before Time" videocassettes.