If you don't want to get under the truck to do this, read on!
If you do want to get under the truck with a cheap pump killing yourself, stop reading! JK

sold my 2004 v8, did this exact thing through the trans cooler and drove 100k miles on valoline maxlife full synthetic. when the truck sold, there was no leaks and the transmission shifted smooth with no issues.

Just got a 2008 garage kept v6 sport and the truck had a bad shudder at 40mph and anything up hill at low rpms. downshifts were rough, hard hitting. overall it drove like shit. over 150k miles with the stock fluid!

There are plenty of posts out there but so many argued the type of fluid and method. I am not here to do that, just sharing what I did.

1.5 hours total time. I did an oil change, plugs, and air filter while it siphoned, also watched a movie... it was a good time.

Every issue disappeared INSTANTLY. shifts like a brand new truck.

original oil was nasty as hell. I think the truck was used for some towing.

you need some clear hose
you need a metal fitment to go in the radiator cooler hose, i used a male and female air tool nipple. worked great

the science: the return line goes back to the transmission towards the pan, the fluid out of the tranny comes from the TC side, which makes sense because it needs to be cooled first.
Laying under the truck to pump fluid in with a small pump didn't make sense because the return line goes right to there anyway.

there are methods of heating the tranny to see the level is correct but its a sealed tranny, if it didnt leak, and your measurement is exact, i doubt it would be an issue. It was proven for 100k miles with my own truck.

1. locate the top cooler line going into the radiator. that is all you have to disconnect in the truck. you do not need to go under the truck.
   
2. you need (2) gallon jugs, both need lines that show exactly 3 quarts.
   
3. connect a clear hose to the tranny hose that you disconnected, that goes to one jug.
   
4. connect another clear hose to the nipple you disconnected the tranny hose from that is on the radiator. its the TOP NIPPLE, NOT THE BOTTOM
   
5. add the clear hose and attach it to a funnel, get it high and hang the funnel.
   
6. start the truck out the driver window, start the truck, watch the jug fill,
   
7. TURN OFF TRUCK right at the third line.
   
8. take jug number 2 and fill clean trans fluid up to the third line
   
9. start pouring into the funnel.
   
10. second time, only do 2 quarts at a time. second line. I did 12 quarts total, fluid was looking good by then. Black to Red with a little black in it.
    
11. the siphon takes awhile, but be patient its going to go in no matter what.
    
12. Thats it! be patient, be EXACT, and forget about the other stuff. Really no need. its sealed. just exchange and be AMAZED.

How simple is that? one connection, 1.5 hours, and you just did a full out exchange! Good luck.