Oil leak

Several months ago I had a new axel put in my '97 Aspire. Ever since, I have had leaking problems. First it was transmission fluid and oil. Now, it is just oil. I can't tell where it is coming from because when I check the dip stick, the oil level is just fine! However, after the car is parked there is a definite oil leak under the front end. I went on vacation for nine days and parked the car in my driveway. What a mess I had when I returned! Still no deminished oil level on the dip stick! No red color. Just a large oily area. Not so large if just left on driveway overnight, but still obvious oil leak. I have taken it back to the garage (Ford dealer) several times and each time they tell me they've fixed it. Right! This time I took a picture of the stain on my driveway! Fortuneately, it cleans up with kitty litter and Simple Green. What could it be? The car runs beautifully and has been well maintained....by same mechanic at this dealership. It's a Ford and they hate to see me coming. Too bad! They say only one mechanic knows how to work on this car. What a toot! It has 67,000 miles on it. Still a baby! Great pick up and okay gas mileage. A workout to get out of a tight spot as there is no power steering. Oh, well! I am not ready to give it up but am tired of hauling it into "the shop" and getting it back with the same problem! All ideas appreciated! I love this car!