I'm not looking for a full system replacement, just a few quarts. What is the easiest/simplest way to get some fresh fluid in the system?

I do have a oil vacuum if that helps.

What engine and what year model?

04 v6

To sum it up in a few steps (do a search to see exactly what is involved).

1) unscrew FILL plug
2) unscrew DRAIN plug and collect the old fluid (try to use a graduated receptacle (pan/bucket/whatever) too see how much comes out. Over the years, people have reported about 3.5 quarts.
3) Screw the DRAIN plug in
4) Pump in the amount that drained out 3.5 quarts if not a little more.
5) Turn engine on and (easiest way) get a scan/OBDII reader to see when the transmission gets to about 100-115 degrees. When it does.......
6) Unscrew the "check plug", some fluid may come out, when it becomes a "trickle", then screw it back on. This indicates that the fluid level is correct.
7) Screw DRAIN plug back on.
8) Have a beverage of your choice.
9) If you want ALL (in theory) of the fluid out using this method, do this again a few times after driving it a couple of days in between drain/fills. Eventually, you will drain out most of the old fluid. This is how I've done so in my 2007 Honda Civic. It currently has 386K miles on it. Original tranny and shifts like new.

I'm gathering up all the necessary hardware to do this in the near future. Sorry if this is overly simplified, but it is basically what you do to do it the right way. If I missed anything, chime in!!

So there is three plugs? check + fill + drain?

Also maybe I'm hallucinating but I swear at one point I swapped ATF in a car by unhooking the cooler lines and running one to fresh fluid and one to a empty container. Was I just imagining that? Seems like it would be a lot easier.

What spec ATF do I use?

Since you have the 2004 V6, you have the 4 speed non-sealed transmission, not the sealed 5 speed transmission, which the procedure posted by @CookieRoe is for the 5 speed sealed transmission, so that's not how to do it with your transmission.

On your transmission you have dipstick which is where you check the fluid level and add new or more fluid, you only have one plug on your transmission, which is on the pan, it's the drain plug so drain the fluid here and reinstall the drain plug and then add new fluid using a funnel into the dipstick tube.

The fluid type is...ATF Type T-IV

Here's link to a writeup on servicing the 4 speed non sealed transmission, it includes changing the filter if you want to do that as well, it also describes how to do a complete fluid flush if you want to go that route as well, if you don't want to change the filter or do a complete flush just skip those steps as well as removing the dipstick tube and just drain the fluid and add new fluid.

DIY: Non-Sealed Transmission Service for 4th Gen T4R's and Prior

Didn't see the reply that he had an 04 V6. So yeah, the procedure I wrote is for 2005+. Well said AuSeeker

Hmm I do not have a transmission dipstick. Pretty sure the truck is an 04 though.

My shifter does go from 4 -> D so.... idk I guess I should re-check the year on this thing.

So I guess this thing is an 05. I've been under the impression it was an 04 for a long time.

I've never owned an automatic in my life until I got my original Hilux Surf (4Runner) and I changed the fluid completely in both the A750F and whatever this has (A434E?) using Timmy the Toolmans fluid exchange videos.

Can't see the point in messing around with these partial changes myself.

I know the common lore is that a fluid change can kill an old transmission but it makes no logical sense at all. It's like saying new engine oil can hurt an old and neglected engine.

What really happens is older and abused transmissions only get a fluid change when it's far too late, and problems starts to show. Then when it dies anyway, people rationalise that it must be because of that. It couldn't be their lack of interest and maintenance.

Do I need to get Toyota ATF or can I use something from Autozone?

Autozone ATF+4

Like, would this be good?

I'm sure I've read of quite a few people using Maxlife over there and it's been fine.

I don't know if they do it over there but I used Motul Multi ATF in the "delicate, must use WS fluid only!!" transmission and it was absolutely fine, although long term testing ended when I wrote it off. It met WS standards. Driveability, shifting etc were all perfect, I made sure I did the level check.

The A343 is an older 4 speed design, I used the remainder of what I had left and a couple of other bottles of fully-synth fluid and again it's fine, although I would expect that as it's specced for the older standard (4?)

I didn't bother changing the mesh screen (It's not a filter as such) as both were/are relatively low miles, as they came from Japan.

Well you definitely have the 5 speed sealed transmission, follow Tim's A.K.A. @mtbtim Lexus GX470 flush video, same transmission and procedure as a 4runner, you need to use a fluid that meets Toyota WS ATF standards, the fluid you posted is the wrong type, don't use it, as mentioned by @SurferUK many here use Valvoline Maxlife with good results if you don't choose to get Toyota WS ATF.

Whatever you do your first step is to remove the fill plug up on the passenger's side of the transmission, DO NOT DRAIN ANY FLUID if you can't get that fill plug loose, once it's at least loose drain away!!

Here's a link to the topic by Tim with his GX470 video posted.

Sealed Transmission Service

Absolutely invaluable tip!