If your vehicle has been in water deep enough to go over the centre of your wheels, then water may have entered the rear axle through the vent and the oil should be changed. Changing the oil on most vehicles involves removing the rear cover, letting the oil drain, cleaning and installing the rear cover using a sealant, and filling the axle with gear oil. It is not a difficult job but can be messy and time consuming if you do not have the correct tools. Gear oil has a lasting odour and the smell can stay on your hands for several days even though you wash well. You may wish to leave this job to your service shop.
Some vehicles have a drain plug for the rear axle.
This makes servicing much easier. Leaving water in a rear axle can cause all the bearings to rust and the differential to fail. The cost of overhauling a rear axle can easily run over a thousand dollars so a little preventative maintenance will help prevent big repair bills. Finally, if you drive a four wheel drive vehicle the manufacturer has expected you will get into deep water or mud at some time with your vehicle. The axles, both front and rear, have vent hoses with one end attached to the differential and the open end mounted up higher on the body.