Bottom Line Up Front: It takes a LOT of pushing & muscle to get my car to roll when it's not started and sitting in neutral. Any ideas about why?

My half-restored 1962 Sport Fury has sat undriven for 5 years, and I've been getting it roadworthy. It has a 727 pushbutton automatic trannie, all drum brakes, and a limited slip 489 rear end. When we went to push it out of the garage, it was REALLY hard - took 3 guys to move it on level ground! I assumed the brakes were seized, so I removed the drums - all 4 drums came off easily, and all 4 wheels spun pretty easily with minimal drag. What the....

I double-checked to make sure the car was not in park, and in neutral. We didn't hear any dragging or weird noises when the car was moving.

So I'm scratching my head about where this friction is coming from....I'm guessing either at the differential or the transmission? Both were rebuilt 5 years ago, but haven't been run - could one of those components be seized up?

Thanks for any ideas.