Wind gearbox slipping shifting

Slipping and unclean shifts definitely point towards lubrication issues. How's the oil level in the transfer case? Have you checked for any signs of leaks around the seals? Also, when was the last time you had the transfer case oil changed? Sometimes old oil loses its viscosity, kind of like how olive oil goes rancid after a while. Checking these basics will give you a clearer picture before diving deeper.

Thanks! I haven't actually checked the oil level myself, to be honest. Assumed it was all sealed up tight like a drum. Where exactly would I find the fill point to check it? And how would I even know if it's the right level? Is there some magic dipstick involved, or is it more of a 'stick your finger in and hope for the best' kind of deal?

Okay, so if you haven't checked the level, that's the first thing to do. The fill point is usually on the side of the transfer case. There should be a bolt or plug you can remove. The correct level is typically when the oil just starts to dribble out of the fill hole when the car is level. But honestly, with the symptoms you're describing and the mileage, it sounds suspiciously like internal mechanical wear, possibly defective gears within the transfer case itself. Gear wear causes slipping, like worn tires on ice. I'd strongly advise getting it checked by a professional workshop. Continuing to drive it could cause even more damage.

Right, that makes sense. Didn't want to hear it, but you're probably spot on. I'll book it in at the garage. Just had a call, and it looks like they've diagnosed it as defective gears in the transfer case, and the repair is going to be around 330€. Thanks for the help!