My plan is to run the early 80 series case with a spool in it. Then use the dash 4x4 button from a fj62. The button locks the tcase into 4wd high, and the floor lever puts it into low. You are still part time that way. Im like you, i am not a fan of the full time 4wd setup. Its why i dont drive an 80 anymore and have a 60 haha. You can also drop 3:1 gear sets in them without machining the case.
I too have second skin stick on butyl, closed cell foam, and MLV in my truck and can hear the whine loud enough for it to be annoying. It whines the most at low speeds where you arent really "on it"
Ive tried rebuilding the tcase 3x, replacing the case with a brand new oem toyota unit. 3 different Advance adapters output shaft bearings. 2 different tcase imput gears. 2 different oem trans rubber mounts. Tilting the drivetrain down more than the normal 2 or 3 degrees. Heavier oil. Everything short of replacing the 4l60e. Stethoscope puts the noise coming from the adapter bearing and spud shaft.
Im sure i could try putting a rebuilt 4l60e in there, and that is what the eventual plan is going to be... but im not even going to try it with the split case because the 80 case is known not to whine at all. The output shaft cutoff is longer for the 80 case vs the split case so you only got one shot at it.