So I've got an 1989 Pontiac 6000 with a 2.8 MPFI that starts up and idles good, but when you put it in gear it stumbles a bit, then once you hit the gas it stalls out. You can drive it if you are REALLY slow on pressing on the gas.
Any ideas?

Check for codes stored in the computer. Also check for vacuum leaks which could be causing it to stall under load.

No codes given by the computer. Checked for vacume leaks, but no find.

Check fuel pressure. How old are the ignition parts?

Plugs and wires recently replaced. Fuel filter replaced last weekend.

Check the pressure; the pump could be getting weak and not responding appropriately when you step on the accelerator.