car wont shift into 3rd

[macajames]

i got a 1995 G20 automatic and it will not shift into 3rd or 4th gear it shifts thru 1st and 2nd fine doesn't miss or anything just wont shift took it to transmission guy he said he needed to rebuild it for $2500.00 but i would rather get a rebuilt trans and put it in myself but wanted to make sure it wasnt a speed sensor or something simple before i tear this thing down