transfer case leaking into transmission

[scavengerdog]

tl:dr. read the title, has this happened to you and did you find something i might have overlooked.

sorry for wall of text. i figured a full explanation might help find problem.

2002 trailblazer, found the transfer case fluid was all but empty just after getting truck, so i filled it. no noises and no obvious fluid leak trails so i figured it had leaked out over time and i would watch for leaks. the transmission was a bit overfilled but shifted fine. also had a slight cooling line leak which i thought would account for it being overfilled... maybe previous owner was trying to keep ahead of leak. so i planned on drain fluid, filter changed, leak fixed, etc. two days later, transmission is shifting harsh and is way more overfilled. transfer case was quite low again... mystery solved, i thought. i replaced the transfer case input seal, drained and refilled the transmission. all is good... a week later i check transfer case... low. transmission... high. i'll likely go back and replace seal... but i'm pretty sure i didn't screw it up. i also visually inspected for cracks in front of transfer case because old seal looked good and was pliable.
online search shows vent lines for trans and transfer case are teed together. am going to look into this soon. i checked service bulletins, didn't find anything relevant. when searching online i find many cases of harsh shifting like i had (with what could have been almost 5 quarts of auto track in transmission), but none of the answers out there had any mention of the transfer case issue. i've had the opposite (transmission leaking into transfer case) problem on a couple of mid nineties blazers. one i fixed and the other i just kept the transmission full and drained off the excess from the transfer case at each oil change. but those blazers ran same fluid in each.