1997 Sierra bucks when accellerating

I have a 1997 GMC Sierra extended cab with auto trans and 5.7 engine.

When accelerating it bucks like it is underpowered. If I manually downshift to 3rd it runs fine. Once up to speed, I manually shift back to OD and it runs fine.

Plugs, wires, low fuel pressure, dirty injectors, weak o2 sensor, all fall into these symptoms of misfire under load.