Thanks Mike - cheers.

Sorry for the confusion. I wasn't meaning 2" diameter. I meant that in reference to having two different thicknesses or a step / change in diameter as it left the cv joint.

I'm not aware of any specific problems with thinner driveshafts. Nobody puts out enough power to worry about that. I've put thousands of miles of track abuse, including using clutch type limited slip differentials, and up to 195/50-15 grippy tires never had a driveshaft problem. If you bounce off curbs or generally drive like an idiot over rough terrain, then maybe you need to worry about it. 

If it ain't broke, don't fix it. The actual driveshaft isn't your problem. One or more of your circlips and/or cv joints are your problem.

Thanks mike...I just managed to push it back towards gearbox with the tips of my fingers.  Don’t recall hearing a click but it’s back.  Still don’t feel confident that what I did ‘corrected’ it from a mechanical/tolerance standpoint, but I need to find out why this side is problematic.

you mentioned that my shafts are early type.  I used to have a 998 on these shafts and now have 1293.  Was there issues with the older shafts...you said they were 2” diameter?

Does the pot joint inner slide freely in and out of the "cup" or outer of the pot joint? Might be that something is binding and not allowing the greasy balls to slide in and out.

Grab the driveshaft and see if you can get it to plunge in and out of the joints-- it really shouldn't while the outer cv joint is properly installed. But if you pop you lower or upper balljoint and pull it out of the top or bottom arm, you should be able to get enough movement in and out to test. 

I'd just pop the cup back in. It should take a little effort while it pops over the circle on the differential stub.

(also, that looks to be an early pot joint axle because it has a step in it. I don't recall seeing the 2-diameter shaft very often)

A few months ago, I was stranded on side of road when my LHD outer drive shaft pulled out from outer CV and boot was still connected, but was disconnected from inside.  After fixing, it looked like the circlip did not do the job and after measuring the thickness of that circlip, we replaced with an original old one and has been fine since.

Now this is happening - the inner CV is pulling away from the gearbox and I caught it by luck today after a quick run...no oil is leaking here nor has it pulled out completely, so I am lucky.

But why is this happening on this same driveshaft!?  Is this a common problem?  I cannot find anything online relating to this being a common problem.

Do you think it could be a bad circlip on the output shaft for the pot joint?  Assuming that both circlips are now the same according to this post.  Or is it something worse - where there must be some binding happening on this side that is overpowering the circlips and putting stress on the driveshaft itself?

I do not drive this mini hard at all and only on weekends.  The driveshafts are original and looked to be in good shape.

Looking for any answers here as I cannot drive my mini in this state.