can't get u-joint out of yoke / off rear drive shaft!

[centuryhouse]

Is there a trick to this? I did this 20 years ago on a Mustang and it was easy. What gives??

I have removed the rear drive shaft on my 72 GMC pickup. I am trying to replace the u-joint mating the slip yoke to the rear shaft. I have removed 4 outer clips that retained the u-joint.

I used penetrating oil. I tried using a socket and hammering the u-joint out. I tried a c-clamp on it. It hasn't moved even the smallest bit.