ive got a 91 k5 blazer with the 5.7 and the 460 auto trans i was driving yesterday and it shifted to 2nd and when its supposed to shift to 3rd it downshifted and i thought maybe if i let off and it didnt do anything my 3rd gear is gone.i was not driving full throttle i had barely any pressure on the gas. but i can put it in 2nd and when i know its supposed to be in 4th i put it in D then OD and itll drive fine until i stop and have to do it again.
please help would a trans flush help or do i need to rebuld it

 

you didnt mention how it was leading up to this. Was it shifting fine with no slip? If it was slipping, likely need a full rebuild.
If it was shifting firm, probably a blockage in the valve body. I would take it to an independent shop that does trans, let them drive it and ask what they think... tell them you are trying to figure out whether to have it fixed or part it out. Will get you the most honest answer.

Trans flushes are most effective at cleaning out your wallet at that age and mileage.

 

turns out the TV cable for the trans was fiddled with and it put the nail in the coffin so it got rebuilt.