water in the rear diff...but how?

Have you got air lockers.

Have seen this with lockers fitted to a TD5.

All my rigs,over the years have had the breather hose run up to the high side of the engine compartment where they breathe air above the normal water crossing level and the air should be more moisture free as the diff/box cools then it sucks in to fill the void. Just remove the breather,buy a fitting to suit diff thread and hose tail on other end(elbow may be best) then also,a hose-tail to suit high end of hose with a female thread to suit the breather.Allow enough slack for diff drop over washouts,kerbs and "zip" tie it all up (use black ones..some white ones are a bit sus) Keep clear of exhaust/manifold (they did'nt when they fitted my ARB to the front..but they were good about it..Gave me some free hose.) I'd like e Defender in my fleet..

check your wheel bearings and seals... pull the axles out to see if they have signs of water too

Greenextreme you have answered your own question. No extended diff breathers is the most likely cause of the problem. Most 4WD users seem to think a snorkel will solve all problems but many don't understand that even water flowing over a hot diff housing will cause the standard breather to "swallow" water. I have seen this problem with three Disco's in the last six months. But the problem is not limited to LR in any way.

As well folk seem to think that bearing seals are impervious but they are not so it always pays to take care and a little time when approaching a water crossing, regardless of depth.

Extended diff breathers will go a long way to preventing a recurrence but it won't guarantee the issue will not return given the issues with bearing seals.

If the problem has existed for a while then a solid check of everything is on the cards as if the oil has not provided full protection from heat and corrosion then you could easily have issues so get it thoroughly checked by an expert

Rob