OK picture this. It's about 3:30AM and i'm on my way to work. I'm just pulling off the motorway and had slowed down to around 70 when this god awful noise starts coming from under the car. And it was LOUD! It was similar to when you hit corrugations at around 30 - 40. At the same time the car starts slowing, not like it just lost power, but like i was hitting the brakes, except I wasn't! I steer it off the road (the noise is still there, and still FAWKING LOUD) and it grinds, quite literally, to a halt. Engine is still running fine so no problems there.
So I get out expecting that a wheel has fallen off (it's happened before

) and do a walk around. Nup, all seems fine there. Have a look underneath. Nup, everything looks normal. Hmm Ok then. I see it's still a bit too close to the road for my liking so I go to push it onto the grass. Nup, not happening. Out of gear, handbrake off, nudge, push. Not a sausage. Hmmmmmmmmmmmm
So I remember on the weekend I was manouvering the trailer up the (dirt) driveway and was having traction problems. Yeah so I had locked in the hubs and not unlocked them. But I have run locked hubs on road before and never had a problem like this.
My first impression, upon hearing the noise is that it's front diff. I'm really not quite sure how at this stage but that's what it sounded/felt like.
Anyway, so I unlock the hubs and push it off onto the grass. Yep no worries. Start her up, 1st gear away it goes, like nothing ever happened. Other maybes are the front output of the transfer has tried to engage itself, or that a busted CV has locked up in the knuckle housing. I've gone through all the possibilities in my head and nothing quite fits yet.
Any ideas?