1997 Chevy blazer 4.3 vortec runs terribly
 

It ran perfectly fine until swapped transmission.
hasn't run right since & no clue why

Doesnt say too much on what your issue is... might want to explain a little more what your talking about...

Option a: The sky is blue.

Option b: The sky is blue with white clouds, and the sun is shining.

I can see the picture in option "b"...

You hook everything back up right? Sensors, vacuum lines, etc?

How does it run terribly? Idle rough, stalls, no power after xxxx RPM? C'mon

You gotta give us something to go on, man

Welcome to the Forum... But yeah, gotta give us much more info to go on lol... There are a ton of helpful folks on here who bend over backwards to try and help others as you will see given time :)