I had the same plm with 4 pinlock kegs and those posts ! I tried heat, soaking, banging , keglube ...
I even sent the socket back to midwest supplies, thinking it was poor design. They sent another, and I still had no luck ... just a weakling I guessed.
Then I hosted a brewing get together and Paulthenurse came to the rescue and, with considerable effort and more strenth than me, got those post off..
my kegs were rescued !
so - do you have any really strong friends ??