My frame must have been a mouse house.

Congratulations on finding a nice MoPar, Shadango, I know you have been looking hard for one for a long time.

I would bang on the rails with a rubber mallet to loosen up the stuff a bit. You might then be able to blow a lot of it out with a tube attached to your compressor (as mentioned above). If that stuff was once mud, either from driving through mud puddles or from rodents or insects, you might need to hose or pressure wash the remainder out of the rails. This is not a bad thing, in my opinion, because moisture gets into there anyway and you need to get as much of the mud out as possible. Give it a good douching.

Then, as mentioned above, dry it out real good with compressed air, wait a week, and then spray some Ospho into there.

Good luck with your project.