Have a leak in the front to rear brake line at the frame rail beside the fuel tank.Can the brake line be removed without dropping the fuel tank?

I don't know that answer.
But if you don't want to remove the tank you can easily tilt the bed up (remove front 4 bed bolts, loosen the 2 rearward, remove license plate bracket, tilt the bed).

Been a few years, but it seems that brank line was attached to the frame somehow, maybe a spring clip. When I gave it a tug all those years ago, I believe it came out without any issues. If you cover the end of the new line (to keep debris out of it), it can be fed in between the tank and the frame pretty easily. Then remove the cover and connect both ends. Then I secured the new line to something near the frame to keep it from rubbing against anything. Must have worked well because the replacement line outlasted the truck and the truck was on the road several years after the line was changed.

This...

There's a plastic clip on the inside of the frame rail that holds the brake line in it to the frame rail, behind the gas tank.
Where the plastic clip is at is exactly where my brake line rusted through from the outside.

On my 99 I lifted the bed up to replace the brake line.

On my father in laws truck they just fished a line through between the tank and the frame rail.

Either way works.

working on old, rusty, stuff can be a real challenge, but i have successfully fished a new brake line between a frame rail and gas tank many times in the past.
it seems removing the gas tank can open up another worm can, because rust can cause issues with mounts, straps, etc.
as was said, cap the end of the new line good to keep stuff out of the new line when fishing the new line past the tank, and you will be ok.
and since you are replacing the main line, you might just as well replace the rubber line to the rear, and the rear's hard lines as well.
then those brakes only need to be bled once and done for many years to come.
just my opinion and life experience with brake lines.
your mileage will vary.

I cut em off where I can reach them ahead and behind the tank and just fish a new line thru between the tank and the frame
Done many of them this way

Same, done a few that way

I had to replace that line on my 2000 2500 Cummins ram 4x4. I had pulled my fuel tank in the past and if I remember right brake line shares clamps to the frame with the fuel lines. I can take a look as the truck is currently in the backyard with the tank out.

You can just run a new fuel line over the tank if you really don't want to drop the tank.