Falls flat under acceleration

[BILL NIVEN]

Fuel pressure under load was 12psi.

Check engine light will come on during the hard acceleration when it spark knocks.

 

[Schurkey]

Install scan tool, look at the data stream.

If it detonates (knocks) under load, I'd be looking especially at the knock sensor readings, engine temperature, O2 voltage and cross-counts, and EGR function.

 

[Phillip Buessing]

Was this issue present before you installed the “new” Tbi unit? Any chance your exhaust is restricted?

 

[kauila]

The old rubber on the vibration damper may have slipped so that the 0 degree reading on the damper is not really zero anymore after someone re-adjusted it. The engine might be able to correct the bad timing enough to make the truck run decently under normal driving, but it probably would fail to do so if pushed too hard as you described.
I'm not saying this is the problem with your truck, but it might be. One of my Chevy trucks has a bad (slipped) damper and I never bothered to replace it. I figured out the general range of where the idle was best and just set the timing in the center of that. It is a strong dependable truck that has been used hard for many years and is still going strong.