Driveshaft install - won't slide forward!

Hey guys, I'm at the end of a few week project and have an issue. When removing the driveshaft, it slipped forward fine so that I could drop it from the rearend. Although it was in the trans very tight, a few taps with a hammer brought it out. Fast forward to tonight, the driveshaft now slips in to the 727 fine and goes up to the point where it sits when finally installed (can tell by the surface rust line at the front and where the caps meet the u-joint in the rear. My problem is I absolutely can not get it to slide forward to allow me to set it in the u-joint!

The trans is in neutral, I've tried rotating it while pushing forward. I smoothed out the surface rust at the front and lubed it. I've given it every ounce of *** I've got, and it won't budge forward. The car is up on jack stands, and I've tried with the rear end fully dropped and fully jacked up. What am I missing here?