Pardon me being honest, but just the way you describe this tells me that you should take it to a mechanic to have this fixed. to get better response than i can give, please state make and model of the car and how many miles it has gone.
it is serious.... meaning that if you do not address this soon, your differential will run out of oil and malfunction. you can drive short distance to mechanic, but avoid longer drives and top up the oil level in the drive before you go anywhere.
The pinion oil seal is toast and maybe the pinion bearing itself. If its just the oil seal, this is a 30 minute job for a skilled mechanic, but if you need new bearings, this can cost a bit and take time.
If this is only the oil seal and you want to fix this yourself, there are plenty of faq-s here on how to do this, like this one:
https://forum.ih8mud.com/79-95-toyota-truck-tech/475982-wrong-way-change-pinion-seal.html
if the pinion bearing has any kind of play... up, down, in or out, you need new bearings and that means taking the 3rd member out and reinstall some bearings. I strongly suggest you have mechanic do this if this is the case.
hope this helps.
