Doesn't seem like anyone's in a rush to answer your question, so I'll try. The term "negative offset" rims means that the mounting surface inside the wheel is inside the centerline of the wheel. It puts more wheel and tire toward the outside of the fenders. The opposite being "positive offset", which means the mounting surface is outside the centerline of the wheel, hence, puts more wheel and tire under the fender. Stock Jeep wheels are positive offset and with no lift on a YJ, 31x10.50's will rub. Especially with sagging, stock springs.
Your diff leak, however, is most likely NOT due to your neg. offset wheels. If the wheels WERE causing too much strain on the axle bearings and seals, it would leak out through the outboard ends of the axle tubes. Because Wranglers are so light, I don't think this is a real concern, however. I've had neg. offset rims and 31's on my Jeep for a couple years and haven't had a problem.