in the past couple months my 3rd gen 98 runner has gotten dramatically gutless. it all started with the check engine light that i scanned and came up with O2 sensor. so i replaced that. then later it came on again with knock sensor. havent replace that cuz i was told that would not cause a problem of being gutless, unless it was messing with the timing which i have determined it is not. however the truck has being getting louder and louder with a slight rattle. suddenly on my way home from work it got REALLY loud and the rattling from the exhaust got worse. on top of that when this happened my loss of power spiked leaving me at a top speed of 45-50 on the freeway when the speed limit is 60.
does anyone have any ideas as to what this might be or what i should do?

By louder do you mean exhaust leak or rattle?

My first guess would be the catalytic convertor. I had one go bad on my 95 T4R. It started rattling after the 'contents' broke loose, and got progressively louder. If they get clogged (as they deterioriate) they can kill power. (I didn't notice any loss of power - it was a 2nd gen and they come from the factory with 'loss of power'!) My son-in-law had a Saturn with a bad convertor and it wouldn't make enough power to hold highway speed. It's like sticking a cork in the tailpipe.