I own a 2004 Subaru Outback 2.5 Auto with 207207 miles, I don't want to get rid of her but I'm afraid I'm going to have to if I don't figure this out

2 days ago, I'm headed to ace hardware and I feel a huge amount of power being lost not all at once, however. I pulled into a parking lot and had my girl pick me up and bring me home
Yesterday after buying a new car I had to pick up the outback before it was towed, I was able to drive it 20 mph home 3 miles away as I was driving it, it was able to reach 4k RPMs but barely got to 20mph. as I was driving it kind of loped as I was moving

I am pretty well versed on how to work on cars, I've replaced both front wheel bearings, ball joints when I bought it about 2 years ago, the cv axles about 8 months ago, the radiator about 2 months ago and I do oil changes every 3k miles.

Low trans fluid.  Classic for it to rev up as the pump isn't circulating any fluid and you didn't mention any noises, additional symptoms, or CEL or flashing AT light at start up.  It could be lots of other things, as this infers more information than you're giving. 

 tremendous squealing under acceleration, a light rattling sound upon startup, no cel light but there is a abs light but that is from a wheel speed sensor missing. no AT light at start up. when it was cold I checked the trans fluid and it was about half, I'm in the process of warming it up to check level properly

 

Just to be sure:

Does the CEL lamp light up during the test-mode on starting up? All warning-lamps should light up briefly when starting the car.

If the CEL lamp is not working, then no CEL problem will show up during driving.

AT Light Flashing 16 times on start you have a code.

Also take a good look at the rear diff and make sure the half shafts are fully seated.  It take only about a 1/4" of outward movement for the axle to disconnect from the drive line.  All that power going no where.

 

3 hours ago, pascullon said:

 tremendous squealing under acceleration, a light rattling sound upon startup, no cel light but there is a abs light but that is from a wheel speed sensor missing. no AT light at start up. when it was cold I checked the trans fluid and it was about half, I'm in the process of warming it up to check level properly

Ah, that’s a good bit different than the first description.  If by “half” you mean halfway between low and full then that’s a nonissue. It would have to be below “Low” to incur issues. 
 

is the sound central, frontal, rear, or does it favor passenger or drivers side?

In addition to Imdews suggestions first, make sure the trans pan isn’t dented even a little bit. 

Id scan for engine codes just to be sure.  

what’s the story on the timing belt?  I’d pull the covers and make sure a belt pulley isn’t about to go volcanic. 

+1 on checking belt and rollers - esp toothed idler.

was steering stiff too?

any overheating?