Did I screw up my transfer case accidentally going from 4H to 4L improperly?

You probably did more damage driving in 4hi on pavement than trying to shift into 4 lo improperly. 4X4 isn't intended for wet pavement. For that you want AWD.

Using 4X4 like that may not cause it to break right now, but you did cause excessive wear. You, or maybe the next owner, will see parts fail prematurely.

Coming from a guy who had a transfer case rebuilt twice in just a few months. The vacuum hoses that locked the center diff on the truck I owned at the time had gotten reversed. When the switch was in 2wd I was actually in 4X4. I was in my early 20's at the time and knew better than to be in 4X4 on pavement, but not experienced enough to recognize the problem.

 

Surely not good and probably less of an issue it being a chain driven case, but she soldiered on for many years.

 

It was a Jeep and you drove around for months in 4x4, making hundreds if not thousands of turns on bone dry pavement.

 

Assuming open axle differentials, the sum total angular velocity of the front wheels must match the sum total of the rear wheels but the individual wheel speeds don't need to match. But yes, this 4wd axle-to-axle constraint results in excess braking torque being distributed to the axle with traction prior to locking up.