how can i fix my 1994 chevy silvarado manual granny low transmision ?

my truck has a granny low 4 speed and lost 3rd gear a year ago and resently lost 2nd gear also, could it be just a problem with the shift tube being worn out or is it possibly some thing more seriouse, it never made any noide when they went out, and will still go in to 2nd and 3rd it just wont do anything, and every other gear work's just fine please help