I made three mods more or less simultaneously; drilled my front radius arm bushes, lifted 3" in the front without changing front drive shaft, and installed a wading kit with extended axle breather for the front, which leads to a position behind the snorkel/on the cage.

I have significant vibrations at 30kmph, 90 kmph and just before stopping.

After driving about 3000 km with this setup, i notice oil leaking from the pinion and from where the breather enters the axle.

What's going on? Have I ruined the pinion seal with the vibrations, is the position of the breather a high pressure area, that leads to pressure in the axle, pushing out the oil, or should I be looking for something else?

I have obviously failed in installing the breater air tight, but could this affect the pinion?

It's a 2001 Defender with about 44 000 km on it, so everything is relatively new, at least in theory.

nope, just classic pinion angle problems. The vibe can also be coming from the rear at low speed. Take off a driveshaft at a time to diagnose.

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So, I should expect less leaks if I fix the vibes problem, which I'm pretty sure will be taken care of with a double cardan front driveshaft?

The rear should be a lesser concern, with only 2"+ lift and the long shaft that goes with 110" wheelbase. Also, with the rear setup, pinion and T-case are still in parallel, which of course is not the case in the front with the radius arms.