Whiney rear-end

Ok, I think I know what the problem is, but I thought that I would ask what you all thought. I just got my 02' XLT back from the dealer. The XLT had an oil leak at the rear diff, right axle. They told me it was just the seal, and replaced it, however I have noticed that above 55 MPH I can hear a distinctive whine when I accelerate, and it stops when I let off the gas. Is this a sign that I have an axle bearing in the diff going? I have heard this before in other types of trucks and such, and it was the bearing. This has the independant rear. Oh, what ratio do I have ( I have a factory installed Class III hitch, and T-cooler, and is it a limited slip, or just an open diff?
Also I had a leak in the front axle (pinion shaft), they replaced the seal there too. IS there something I need to be aware of with the vehicle regarding the driveline. Luckly I purchased the best Ford warrenty, so if something really goes, they will have to pay it, less my $100 deductable.
Thanks in advance. :rolleyes:

My 01 Mountie had a chatter to it when I turned. I added some posi rear end additive to it and it cleared right up.