2000 K3500 trans or electrical issue

[SLEA]

So I have a 2000 K3500 dually, 454 EFI and 4L80 trans that started out slamming from OD to drive harshly like it didnt know where it was supposed to be, I changed the TPS sensor and it seemed to help a few days and then to current state where now it idles perfect, will rev up perfect, AT 1st and 2nd is perfect and you can be easy or give it heavy throttle. 3rd it falls on its face and backfires with no power seems like its loading up with fuel. It still wants to flip flop from OD to drive also but not as much it just is very hard when it does.
I changed the output VSS but not the input yet, could it be the input VSS or maybe the ECU. A full time mechanic friend said it sounds like the transmission but i'm not so convinced. I'm hoping one of you is a bit more familiar with this, also it does not have a current ECM code, I havnt drove it in a while to know if it gets a temp code during this process it was so harsh shifting and backfiring I was worried it was going to tear up something.