95 buick century tranny problems?

[Andrew]

hi i have a 95 buick century that i drive daily and its never had any problems until one mornin i went to drive it and if i put it in drive it seems to be holdin back takin full throtle to go but if i drop it down in first and manualy shift it it goes fine my uncle thinks its gettin stuck in two gears at once the fluid needs changed bad and when it started doin this the check engine light came on and has stayed on and the traction control blinks on and off now if anyone has any info on what it doin or how to fix i would appreate it thanks andrew