How do you do if your 1987 Toyota truck jumps out of first and third gear but 2nd 4th and 5th work fine?

IF>.....you have lots of mileage and wear on your tranny,,,then it sounds likely that the "brass synchronizer rings" on your 1st and 3rd gears are worn. These brass rings ride on a machined smooth surface on the front end of your transmission gears (look in a tech manual for visual descrip.). they have teeth on them to match the synchronizing teeth on the front face of your transmission gears. Their function is to create a smooth shift from gear to gear in your transmission without creating any grinding or wear on your transmission gears. The shifting is done with a shift fork in your transmission that is attached to a shift hub that grabs the brass rings and synchronizes them to the teeth on the gear you are shiftin into..smoothly. The reason it is able to do this without any grinding, is because brass is a softer material, and yields more to create less resistance. However, brass, being soft, will wear easier than the case hardened steel that the gears are made of, thus the teeth on the brass rings become worn after time, and need to be replaced. The worn teeth on the brass ring are unable to hold the main gearshaft and gears in the tranny in place, so the shift hub and brass ring "jump out" of gear. Now..the big problem is,,,,now that you are thoroughly confused due to my inability to write effectively in short simple answers...how in the world will you be able to even understand this paragraph of jumbled, monumental and technical B.S.!!!!!!!! ( i hope you can though, and that this answer sheds some light...even if it is the wrong diagnosis!)..good luck!