Tranny bleeding all over the place

Okay, I am no ace mechanic, so some of my descriptions may be more descriptive than technical. That said, I was pulling a trailer full (appr 1000 lbs.) of household goods with my '98 Sonoma, which was also loaded with appr. 500lbs, through the mountains of Pennsylvania. The tranny started shifting a little funny, dropping from 4th back to 3rd when I let off the gas at my cruising speed. About halfway up a really long grade, I see blue smoke behind me. I pull over to discover a metal cap on the bottom of the tranny case which has popped open and is letting all my tranny fluid spill out. It is a small metal cap which has 3 legs that slip into a hole under the tranny case. Without some type of seal, it will not hold any fluid. Any ideas how to repair this? Is the transmission hosed? Is this something I can do or should I take it to a repair shop? My truck is a '98 GMC Sonoma with the 2.2L 4-Cyl engine, ext. cab. Thanks for any help!

Smaug

Sounds like the inspection plug in the bell housing, this is not why the the trans is leaking, you may have overheated the fluid and it blew out the pump seal or vent. After it cools recheck the fluid and add Dexron 111 as necessary and see what happens, kinda overloaded that drivetrain with your task.