Drivers side rear locks up really easily when I hit the brakes.

Check the opposite side to see if it works,and also the fronts.
If the fronts are not working properly, it will require a lot more pedal pressure to stop the car with the rear brakes.If only one back there is working, then the tire gives up and skids.
and Ima thinking the P-valve is defective.If that valve is working right, the rear line pressure will be reduced to about 75% of the fronts. But this does not translate to the rears doing 75% of the work. Typically the rear brakes do 15% to 25% of the work on a DB equipped vehicle. So If you can skid a rear tire, you would need to be applying a LOT of line pressure. And if you are pushing that hard and NOT skidding the fronts, I would be looking to a problem in the fronts, and or a defective Pvalve.
That's in addition to the left/right imbalance,at the back.