What exactly did this? (trans help?)

[Donut]

The rustbucket in my sig is the problem child.

My tranny is toast (I believe) no forward gears, reverse only.  I have started digging into the truck to get ready for a tranny rebuild and motor swap.
I drained the transfer case and noticed it was a gear oil-atf mix!  I can only assume the tranny was pumping atf into the transfer case.  Could anybody tell me what is a likely cause of this?  And maybe any weak spots in the turbo-350 I need to address when i have it apart?  (It's a plow truck, not a high HP street rod.)

[VileZambonie]

If the input seal is gone on the transfercase it will drain out of the trans into the transfercase. Try topping off the A/T and see if it gives you back your gears. If it's good then just drop the transfercase and replace the seals.

[Donut]

Thank you kind sir.  I'll give it a shot when everything goes back together.

[DirtyLittleSecret]

I'll add that my old 1965 Jeep/Kaiser did just this about 5 years ago when my seal went out.  Pulled the x-case, drained the AT, fixed the seal, and refilled.  No problems since.  A pretty common situation it seems.  Does anyone know whether there's a bomber seal for this problem?