I would suspect that you need a new oil separator and lines. The rear main will only "blow" when there is a lot of crankcase pressure, and that you do have.
In the mean time, might I suggest that you pull the dipstick and just let it sit on top of the hole without engaging the O ring? In other words, loose. That will allow excess pressure to go that way, instead of the RMS.
The damage may already be done to the RMS if it is leaking profusely, and that will require removing the tranny, $$$.

Klaus