1985 300 SD - Trans won't shift past 3rd
 

My 300 SD won't shift past 3rd gear. It shifts fine for through the first couple gears and that's it. I also have to keep it in 3rd or it just revs up really high but doesn't move or it jerks back-and-forth into third as it slows down. Also, my tach isn't working. The trans fluid looks good. I was thinking it may be a speed sensor or RPM sensor, but they aren't cheap parts. I would like to know what you think before I start replacing expensive parts.

Thanks,

Chris