Firstly Alfa Workshop have a guide on how to remove the end casing
I don't understand why they say to support the gearbox, the casing came off mine without touching the bolts for the mount. They also don't tell you to drain the gearbox oil, that's the awkward to get at hex plug on the wheel side of the gearbox. Make sure you can undo the top plug to refill it before you drain the oil. The drain is easier if you cut a suitable length off a 12mm (I think or maybe 10mm) allen key, slot that in and turn it with a spanner.
Once the casing is off you will see the ends of 2 shafts. The nut on the one nearest to the front of the car has possibly come loose and caused your problem. Mine was so loose that the rollers had come out and the inside of the hub had worn. Eper says it would be about £40 from an Alfa dealer but scrapyards are closer for me and the same part cost about a fiver. I had a wander round looking for Alfa & Fiats with the same gearbox with the endcasing that comes off before finding the Fiat Weekend. The Fiat has a different ratio on the gears but the hub is the same. A new nut on the end of the shaft properly punched down is essential.
The first picture shows what you see when you take the casing off, the second is the worn hub I replaced. DON'T LOSE THE SPRING LOADED ROLLERS.
The Alfaworkshop link above tells you how to put the casing back on.
If you don't feel confident with any of this get it booked into an Alfa Specialists, you won't be able to drive the car if you get stuck half way through. Work slowly and methodically, I've probably made it sound more complicated than it really is.
The bolts for the casing are various different sizes so I'd suggest drawing a quick picture on a sheet of cardboard and sticking the bolts through it so you can be sure which one went where.
I'm sure there are other ways to do the same job but the above is what I did and it worked for me. If I'm wrong somebody will be along soon to let you know
Hope you have a garage to work in looks like it's going to be cold this weekend.