When i did my fronts i soaked it with PB Blaster first.... the nut at the frame end likes to freeze to the steel line and can twist off easily... Be careful...
You can unclip the retainer at the frame and pull it through the hole to access the line easier. Then while holding the steel line nut, spin the rubber hose to unscrew it. Once removed use a butane or propane torch to heat the flare nut on the steel line. Don't Heat The Nut While Connected To The Rubber Hose. It will swell and then burst usually in your face with hot brake fluid.
Most likely the flare nut will be rusted to the line and not want to turn. I've had decent luck undoing the hose at the caliper and unscrewing it from the flare nut. Then screw the new one on while again holding the flare nut stationary. It may not line up the way you want, but it sure beats fixing the steel line!
If I remember right, the rear line is attached to the axle by the breather bolt, with a steel line coming out of each side to each wheel. The frame end is clipped to the crossmember with a flare nut just like the front. Be careful with the breather bolt, I broke mine trying to remove it. JBG has replacements, and I went around with the hose like zip tied in place for a week.. not great but I needed the truck. I'm pretty sure at least one of the rear lines twisted off.
In short, if you don't have a flare kit and some brake line, make sure you have alternate transportation until you can get it fixed, just in case something goes wrong.
The thing is tho, if the nut is rusted to the line that bad, chances are the line's integrity will also be compromised - I'm usually able to break the nut loose by heating it slightly with a small torch, but if even that proves futile and I have to get her glowing orange to make her let go then I just replace the whole line - really it is not that much extra work, and you end up with a brand new system that you know for a fact will handle any pressure you throw at it for years to come.