I just got my first bronco it's a 91 with a 5.8. I put it in 4wd for the first time today and it started dumping Trans fluid. The only spot in saw it leak was from the rubber cap on the bottom of the tranny. Can I just get a new cap or seal the current one? Or is the tranny shot?

The black cap is where it was leaking

 

The black rubber plug is a closure for an access point. With that plug removed, and the engine crankshaft turned appropriately, it provides access to the Torque Converter's drain plug. The TC drain plug is a small pipe-threaded type plug.

Be sure that what you see leaking is indeed ATF, and not engine oil from a bad crankshaft rear main seal.

If it is ATF, some possibilities - TC drain plug leaking; TC itself leaking via crack or damage; or trans leaking out the front of the case via the input seal, or the pump gasket.

All of these possibilities except a leaking TC drain plug, would require removal of the trans to remedy.

Considering the work and $$ involved to do so, I would recommend further investigation and diagnosis to be sure.

Something you could do, is to remove the metal inspection cover on the front of the trans' lower bell. Wipe out the area with paper towels real good, including wrapping them around a screwdriver or something to reach under and around the Torque Converter as far back as you can, to wipe it all clean of ATF and any grime. Then re-install the metal inspection plate, and drive it enough.
Then take off the plate again, and see what it looks like. ATF that is coming from behind the TC is most likely the trans input seal or pump gasket. ATF thrown around all over could be the TC itself. Engine oil instead of ATF would be a leaking rear main crankshaft seal.

That's a simple thing you could do.

You also need to be real sure that it is coming from inside, and not just ATF from a leak above running down the outside of the trans case and dripping off the lowest point, the bell of the trans!

 

It wasn't leaking after I got home. I put more ATF in and ran it through the gears l. Nothing else was leaking and the dip stick read that it was good. I'll drive it though and see what happens. If it leaks again I'll do what you suggested to properly diagnose the problem

 

AHH. The old "onboard elves fixed it" repair. Never seemed to work for me.