OP do this


Park on perfectly flat ground.
You should be able to access the top of the gearbox - and remove the standard fill hole ( 17mm headed bolt) on the side of the diff housing. Remove this hole , and IF overfilled gearbox fluid will pour out until the level is EXACTLY right. Let it sit for a few minutes until the last drip drip drips , then retighten. Your gearbox fluid is set right.