My new truck has Engine Oil Leak?

[LovNmyBoyz]

I just bought a 2001 Dodge Ram 2500 SLT 4WD 4X4 Sport. I was reading through the paper work on the last oil change and it reads under comment from mechanic:

REAR DIFFERENTIAL LEAK and ENGINE OIL LEAK.

Anyone know of the average cost to fix this? I was unaware and the seller didn't tell me about it so I am thinking it will be a pretty penny.
I honestly wish I knew more. I am not used to a truck as I have driven a car for years. That is all the paper reads. I hear a Thumbing sound in the rear as I drive (Nothing really really loud, but noticeable.) and in the front. Maybe my tires? Only the back right side looks like a tire does when you need a front end alignment. My boyfriend drove it and said there is nothing wrong with it, but my gut is telling me different.
Rear Differential Fluid Leak

 

[JerryS1]

I am afraid you will need to be more specific as to what is leaking. There are quite a number of things which could be leaking, and dripping oil. Anything from valve cover gaskets to front or rear main bearing seals. Cost to repair could be as little at $10 for an oil pan gasket that you might be able to put in yourself, to several thousand dollars for an engine rebuild.

Sorry I couldn't help more.