Why do you have to disconnect the front drive shaft if you leave the truck in park when you do the flush?
You dont. You can brake the line closer to the radiator.
Thats how I did mine. I got a two liter coke bottle, pored in two quarts of fluid, marked a line on the bottle. Then I drained the pan, refilled it, started the engine.
With a hose in the bottle, I'd watch the old fluid fill the bottle to the line, stop the engine, dump the dirty, add two quarts to the trans and start all over, when I got to 16 quarts, I hooked up the cooler line, than started the engine and topped off the trans.
I used 18 quarts as to make sure I got a good flush. I believe the trans holds 16qt.