I took a long (six feet)
steel bar and placed one end of it perpendicular to the front of the
tongue, then wrapped a chain around the intersection to fix it. I then
pulled up/pushed down on the other end of the bar to twist the movable
part of the tongue. Simultaneously I applied fore/aft force, though it
would have been easier with more people and/or using a tow vehicle. The
combination of torsion and axial force gradually loosened it.

OOPS! I forgot to mention one tiny detail in my earlier posting: before
you do this, remove the bolts that secure the inner and outer tubes of
the sliding trailer tongue together. Most trailers I've seen have a
little play (i.e., you can rock the coupler back and forth a few
degrees either way) but you don't want to really lean on the six-foot
lever without removing these bolts. Having done so, you can actually
rotate the inner tongue a full 360 degrees (at least on my Cobra),
which is a good thing to do every once in a while to distribute the
grease more evenly. Or maybe it's just good for the soul.

Chip Bearden
ASW 24 "JB"