85 s10 with high speed problem

my 85 s10 1.9 2bbl carb 4spd manual just out of the blue the other dy refuses to get up past 55. 1st and nd gear are fine but shift into 3rd and the truck will refuse to get its rpms above 3000 and when it reaches 3000 it has a very slight back and forth buck. in 4 gear same problem only i cant ge the rpms above 2500 and the same buckin happens. im thinkin the dif may be a problem but im not sure.