Transmission leak when parked up-hill?
 

Parked the truck facing uphill for the first time this week. It's a pretty steep hill and when the girlfriend went to move it, it just rolled backwards.

I investigated and found it was 1.5 quarts low in the transmission with a pretty decent puddle underneath.

Doesn't leak a drop when on level ground or with the nose down-hill.

I just briefly checked the pan and it didn't look like it was leaking (I recently put in a new Moroso pan gasket w/ RTV.) Where else would it leak from while on a hill? Tranny cooler and all of the lines are new and it seems like if it was the dipstick tube, then the opposite would be true and it would leak when facing downhill?

The tailshaft...

Could you provide some more information about what kind of truck it is? Like the year, model, engine size, trans type(I assume it is automatic, i.e. "dipstick tube, pan gasket" from your post), 2x4, 4x4, etc.
If there is no obvious evidence of where the fluid came from, again I'm assuming the puddle of fluid was a "red-ish" color (ATF), then the next time you park the truck up hill like that take some cardboard and put under the vehicle to see the general area it is coming from BEFORE you move the truck.
I'm not trying to being sarcastic. The more information you provide will help us to diagnose the issue, without us physically being there at the truck. Sometimes it's hard for us to see the problem vehicle and what know what you are trying to describe over the internet connection.

I totally understand. Sorry!

1977 F-250. 460 w/ a C6 and np205. 4x4. Fluid was def. ATF.

Ok, so the truck is a 4x4. Is there any oil/dampness around the rear transmission mount area (where the transmission mates to the transfer case), or the modulator valve (little pod looking thing on pass rear of trans with vacuum line going to it).
Also do you know what kind of fluid the transfer case is serviced with? I have seen some serviced with gear oil, and I have seen some serviced with ATF. Check the level of the transfer case for an "overfilled" condition (pouring out of upper plug hole). If the rear seal in the transmission goes bad, I have seen them fill the transfer case up enough to leak.
Hope this helps enough to get you started chasing the leak. If that turns up with no results, then try the cardboard trick to narrow down the location the fluid is coming from.

This is a divorced transfer case and I just rebuilt the transfer case and know it has gear oil in it and doesn't leak.

I haven't had a chance to crawl under truck to look at the output shaft on the tranny yet, but I suspect that is what is leaking. I'll keep you updated!

Tail shaft seal. Good news is that it's an easy fix.
Drop the drive shaft from the Transmission to the tcase.
Pull the yoke off the transmission
Pop out the seal
put a new one in
Check the yoke for wear around where the seal rides
If it's ok reinstall everything, if it is worn replace it (I'm not sure if you can get a speedy sleeve for it)
Problem fixed.

Thanks! Sounds easy enough -- any tips for pulling the seal? I found another thread that said there was a bushing that you have to remove the extension housing to replace.... Think this is necessary?