It's not really a differential, just a shaft driven rearend. You probably have a leaking ring gear seal. Cheapest way to fix it, is to buy a trike take off rearend and swap it out. Takes about an hour and most trike take offs have 0 miles on them so it's almost brand new. I wouldn't ride it much, that gear oil is slick and it's getting on your tire, you don't want to go down.