2000 Blazer w/Trans problems

A 2000 Chevy Blazer it is an automatic, but will not move in Drive. It will go if you manually shift in 1st gear to 2nd gear and will then you have to place in drive for 3rd and then Drive gear. Basically the Blazer will not "shift or down shift on its own."

Had a mechanic look at it briefly, he said we need a whole new transmission as it is the planetary gear, and needs the whole transmission rebuilt.

Another mechanic said the computer just needs to be "RESET" .

Before I put any more money into it, does anyone have any suggestions?

HELP...:confused:

Thanks, B