I have a 1989 GMC full-size Jimmy. I am needing the steel brake lines from the master cylinder to the junction box. Can I use the lines off a full-size truck?

It should work; otherwise, buy additional brake pipe and a double flaring tool and make your own.

Correct advise.
Use the proper flare tool and save a lot of money.
Also, be patient when bending brake piping: you cannot use it if it kinks.