Looking for Help: The left clutch on my D2 5U will not release unless I’ve got the left steering clutch lever pulled, and I have to press the left Brake pedal to break the clutch free so that I can turn and it’s quite abrupt but once I get it broken free, I can feather the clutch to steer more or less aggressively. Anyone got any suggestions on how to adjust or fix this problem?

You just described how to steer a D2 which is correct. But I know what you're trying to say. The tractor will only steer slightly if it's under a load like pulling something or going uphill or traveling a greater distance for the released track to slow down under friction enough for the driving track to pass it up. Going down hill is a little confusing because if you want a slight turn; you pull the opposite steering clutch lever so the released track freewheels faster than the driven track passing it up.
To keep from turning too abruptly ; Don't push so hard on the brake pedal.  But what may be happening is the splines in the steering brake drum may have grooves worn in them possibly rust preventing the clutch pack to release smoothly. I have a Twenty-Eight with a similar problem. When I am pulling my grader with it under load; I have to pull the clutch lever back and push very hard on the brake pedal for it to release and of course, over steer. If I'm only driving the tractor without the grader it releases easily.

Check the amount of freeplay in both steering levers; 3-4" is what I prefer. Normal to have brake application to steer without a load. While you are at it, adjust the brakes as well.
Lube everything as well. Lack of use makes steering difficult also. JM

I've had one that had a very heavy build up of grease and gear lube from years of neglect. Wash the compartment out with diesel as covered in the operation and maintenance manual. The goo would cause the clutches to stick together. Once cleaned they operated properly again. Just one more thing to try