I'm trying to figure out which is the best replacement brake line to use for my 2003 xlt single cab short bed

 

Welcome to the forum

You will need to be more specific
There are 8 different brake lines on RABS models and 10 on 4WABS(4 wheel ABS)

 

Well it's the hard line that runs along the driver side of the frame. It's connected to the soft line that runs to the diff. The truck is just rwd

 

Yes, the rear brake line often rusts out, usually where you can't get at it without dropping the fuel tank, lol

Rangers used 3/16" lines

I have replace that line on a few Rangers, I think I used a 4ft pre-made bendable line to feed through the frame rail and connected it at the front on the RABS unit
Then used a shorter bendable line and coupler to hook it to rear fitting
Put tape over the end of the new line before feeding it thru, to keep it clean

You should disconnect current line at RABS unit, and plug the hole at RABS unit or it will gravity bleed ALL the brake fluid out of the master
And disconnect the rear of same line and cut the line to get the fittings, so you can match them, the threads and size
Ford used different threads and sizes even at different ends of the same line

You won't be able to pull the old line out, its fasten to the frame
I never fasten the new line to frame and never had any issues