I have a 2000 blazer 4x4 4.3, starts, runs and drives fine. No CEL on. 129k.

It loses all power If I put the pedal to the floor to accelerate from 20-55mph It seems like a fuel pump, but I can hear the pump turn on, and it starts right up and will travel down the highway with no issues. It just falls on its face when I get into the throttle under driving conditions. I just changed the fuel filter today and am still having the same problem. It will accelerate fine if I ease into the throttle, but its hard to get it going if your in a hurry to get up to speed. The weird part is that it will run fine when its parked, I can get it to 4,000 rpm with no hesitation.
I'm just out of ideas and would like some input before I go and put a fuel pump in.

Any opinions out there?

 

Yep, could be fuel pump; do the pressure check as described in the "sticky notes" above. Also, how old are the O2 sensors and catalytic converter? One way to test the cat is to remove the pre-cat O2 sensor and take for a short drive. But you will need to make some kind of deflector for the exhaust coming out of the O2 sensor bung so as to not melt the floorboard. This will also set the CEL as the O2 sensor will not be sending a vaild measurement to the PCM. Requires this code to be cleared.

Good luck.

 

That could be a bad cam sensor. I had similar symptoms and that was the issue with mine.