So far, I haven't been confronted with this problem, but from your description, it would be my guess that the units are being held in place by burned carbon.

If it were mine, I would try something like penetrating oil, diesel fuel, etc. to soak into the carbon and soften it. I would keep adding the solvent and, after giving it time to soak through, again try moving the frozen part. It may take a while and repeated efforts, but hopefully, it will eventually come out.

Hope this helps. Let us know how you come out. Good luck.

__________________
Ted
1979 240D
165,000 miles