Squeak/chirp from rear end

Hello, people! I'm a long time reader, first time writer here. Maybe someone can help figure this out! It's a chirping noise that seems to be coming from the area where the driveshaft meets the rear end. For the first mile of driving, it's quiet. Then I'll notice the chirp at around 15 mph and as I speed up, it gets faster until it's a solid kind of whistling sound. After I pick up more speed it seems to go away (at about 40mph), or it could be road noise drowning it out. But it definately kicks in once it gets heated up. About 3 months ago, I had my stock gears put back in and the shop put in all new bearings and seals as well. I've been under the car and everything checks out normal. Even jacked it up and ran it in 2nd gear to try to pinpoint the sound. It's just more annoying than anything! If anyone has any ideas, feel free to write!

There's the possibility of U joints, but the car only has 106k miles on it. The sound stays the same if the brakes are on or not. I originally thought it was the brakes cuz I had my left rear caliper sticking so I just replaced it this week. The car rolls better but the noise didn't change. I hear it resonating thru the whole rear end when I jack it up and let the wheels spin. I'm gonna take it to a trans/rear end shop and have them check it out in a little while. I'll let you all know what it turns out to be in case anyone else encounters this noise that's been driving me nuts for the last week and a half! Thank you guys for the quick responses!

Well, I just had the U joints replaced and it still does it. The old ones weren't bad, but they weren't great either. It's not clutch, shocks or brakes. The shop had it on their lift and "drove" it. We could clearly hear the noise coming from what sounded like the rear U joint or from the pinion bearing. I called the shop who did the rear end and said it's most likely from the outside of the gearbox, cuz noting inside would make this particular sound. Anyway, I'm going to take it there next chance I get and they're going to listen to it. Hopefully it's a simple easy fix. Until then, there's always that Mach 460 stereo to drown it out! haha I'll update when I finally figure it out... I'm still open to suggestions.

UPDATE! I finally got the noise to go away. The U joints the shop put in for me were faulty somehow and were causing vibration in the gearshifter as well as a choppy noise on the freeway. So I decided to go with a Ford Motorsport aluminum driveshaft. It makes the car feel lighter and smoother, I'd recommend it. So I still had the noise after all that. Then after taking a long drive in it, 200 miles round trip, the noise had stopped. Over the weekend I found some gear oil on the right rear wheel = leaky axle seal. I remembered checking my differential oil a while back and noticing that there was air pressure built up in there when I pulled the fill plug. I checked the vent and found that the rubber piece in there was stuck and not letting any air pass through it. All I did was take it apart and remove the stuck piece then put it back together. Now I have no more noise and no more leak. Just thought I'd share this cuz it's something that most people wouldn't even think of. All I can figure is the pressure build up was causing the noise somehow. Of course, this would only happen to me!