Driveline "bind", or drag?

For some reason, I can barely roll the car forward or back by hand all of a sudden. It was fine until yesterday when I drove it out of the garage and turned it around in the street. Thats when I noticed the problem when I pushed the clutch in and the car stopped rolling by itself.
If I drive the car, it feels like the brakes are dragging, and the car will roll to a stop. I verified the brakes aren't the problem, and I suspect it's a transmission problem. The car has an 833 4-speed trans. With the engine off and trans in neutral I can barely roll the car forward or back by myself on level ground, and I used to be able to do this easily in my garage.

Any idea where to look, or how I can check the trans for problems without pulling it out of the car? I know there's a trans oil leak thats pretty bad, but I only drove this car maybe 3 miles since I got it running about a month ago, and never even got on it. I basically just idled it around the block shifting it at maybe 1500RPM to see how it drove. I hope I didn't wipe out the transmission by driving it low on oil. It's not empty, but it's pretty low as it's dripping out of the reverse lever seal.
ANY help or recommendations on how to diagnose it are greatly appreciated.

Car is a 69 Dart with 440 and 4-speed with 8-3/4 rear.

George