If you've never changed out the lube, I'd do that and put in a bottle of GM's Positrac additive. Worked for me.

 

Changed the rear end diff. fluid with the additive. Also got both halfshafts out and removed the old U-joints (very ugly!). Right now I am cleaning up the area and looking to see what else need to be changed. I did notice that on the driver side trailing arm that there has been metal on metal contact with were the halfshaft connects to the wheel. The trailing arm looks shot anyway, it's rusting out at the bottom. Could that possibly be a wheel bearing issue. I haven't checked those out yet. I ran the car without the halfshafts in, put in gear to see if the vibrations still occurred at around 40, nothing and the rear end sounded really smooth. But when I tried to put it in park, I heard gears grinding in the front area? Could that be the starter and flywheel, trans?

 

I suspect since you had most of the drivetrain disconnected, there wasn't enuf friction to stop the transmission from rolling and maybe what you heard was the Park pawl hitting the teeth. If you've ever put a vehicle in Park before it was completely stopped, it sounds the same as a bad shift on a manual tranny.