Not sure what brand or type of boom it is but if you can remove the locking assembly, slide it back out of the way, then pour some boiling water down between the arm & extension.
Let it soak for a couple of minutes then put your foot inside the extension and pull the stuck arm, the other should move by it's self.
If it's still stuck try the vinigar or CRC and repeat the back breaking bit.
Last resort would be to lightly hammer (with a soft mallet) around the arm where they are stuck, if it's sand it should crumble enough to release.
Don't hammer a Carbon Boom, all the other options should be ok though.
Nothing wrong with fixed length booms anyway