Hello all. I have a 77. My driveshaft popped out if the transmission while I was driving. I immediately pulled over. I was lucky enough that I only had to replace the u joints in the driveshaft. When I started the car, transmission fluid streamed out from under the car. It was raining pretty hard at that time, so I couldn't see the exact place it was coming from. It doesn't look like it's coming out of where the driveshaft connects to the transmission tail shaft. But I could be seeing it wrong. Any thoughts at all? I. Know it's not the best description of the issue, but any help would be much appreciated. Thank you. Have a blessed day

You really need to get under it to see exactly where it is spewing from, but it is quite possible that the rear seal was damaged when the driveshaft came out.  This is reasonably easy to replace.

The other possibility is that the transmission case was cracked with the driveshaft flailing around.

I have to assume it is an automatic transmission....a 4 speed would not leak like that. You definitely need to get it up and see where the fluid is coming from. As Bill mentioned, it's possible there is damage to the tailhousing of the trans, however,  you say it runs out when you start it up. That means the leak is pressurized fluid leaking, not just a leak from a seal, or crack in the tailhousing. Does it ONLY leak with the engine running, or is it leaking any time?