1989 Dodge Dakota Engine Backfire/No Power Under Load

Hello. I have an '89 Dodge Dakota 3.9 V6. It has lately developed a backfire and loss of power.

In October 2008, my smog pump failed and I cut the belt on it. I drove it until the beginning of June 2009 without the smog pump.

I stopped driving it because when driving down the highway in 5th gear at approximately 60 mph and about 1800 - 2000 rpm, it would slow down drastically when going up even the slightest incline. When I would attempt to give it a bit more gas, I would get no engine response and a series of backfires. The backfires appeared to be intake backfires. I would have to drop a gear (sometimes two) to keep the truck moving, but, even at a lower gear, would not be able to accelerate up the incline, and would only be able to gain speed after the road leveled off. Sometimes, the backfire occurs when leaving from a dead stop.

I have since replaced the entire air intake system (smog pump, check valve, diverter valve), the distributor cap, rotor, and spark plugs. Same problems.

I have an MSD 6AL ignition system on the truck, a K&N Air Filter and filter box. Spark doesn't seem to be the issue, as the engine seems to idle and run without a problem. The air intake system and the catalyic converter are working properly (just passed emissions with no problem). Also, I just replaced the fuel filter and the fuel pump about a year ago (fuel pump failed, replaced the fuel filter for diagnosis).

Any idea what may be wrong with my truck? Thanks!