Axle seal leak 2004 Ranger

While getting a free oil change at a Ford dealer I was informed of a leak in the left rear axle seal on my 2004 Ranger (2wd 50,000 miles) and told it could be fixed for $350-400. I thought it might be a scam to generate some dollars on a free LOF. But sure enough on later inspection I could see a small amount of oil on the wheel, but not enough to reach the tire or the ground. It had been there awhile and was covered with dirt. I've never come across this problem before. My questions are 1st, is this a serious issue or could I monitor the fluid level and let it go for awhile? And 2nd, is this really a $400 repair?