The 6L series transmissions and 4L80e have a rear case seal however the 4L60e/700r4s don’t, interestingly enough. The 4l60e and 700r4 rear output is mostly sealed off via the rear case bushing (a sealing bushing as opposed to a lubrication bushing) and rear planetary ring gear hub’s journal but excessive fluid can leak past and into the extension housing/trans-tcase adapter if that bushing is excessively worn. The extension housing/adapter takes a square cut rubber seal on the perimeter and there is a seal on the t-case side that prevents fluid transfer but if it’s also worn, transfer can occur. That bushing, along with all the others should have been replaced when the transmission was rebuilt (not saying it was or wasn’t, have no idea in this particular case)
6l80e
View attachment 399173
4l80e
View attachment 399177
4L60e (bushing removed, new bushing not yet installed)
View attachment 399178