There's this squeak that always happens at low rpms basically when ever I'm accelerating, decelerating, and especially when I'm in reverse. It also always stops when I stick the clutch pedal in, so it seems to only happen under load. It's coming from the under side of the rear of the car. I was told it could be a few things. Such as the u joint, the diff fluid, and the carrier bearing inside the diff (it has an lsd btw). So I put some wd40 on the u joint and it made no difference. I checked the diff oil and it seemed clean and it wasnt low at all. I dont really know how I'd check the diff bearings, so I haven't ruled that out yet. I also checked the brakes to make sure it wasnt somehow that. It also still makes the noise when I have the car on a lift so there's no friction with the ground. Also it's not the belts as those have all been replaced recently.

I'd rather fix what ever it is myself, but I dont want to tear into it until I know what it is. Thanks guys.

 

Sounds like you're on the right track. It's a process of elimination. I would first suspect the u-joint before the differential. The squeaking noise you hear is often a telltale sign that a u-joint is bad. If the grease in one of bearing caps has dried out, than metal-on-metal contact between the bearing cap and the trunnion will cause a squeaking noise. WD-40 won't do you any good. The u-joints on the stock Mazda driveshaft are staked-in and not replaceable. Maybe it's time to replace your driveshaft. Mazdatrix sells both stock and aftermarket driveshafts. Rockauto sells them too. They'll run you between $350-450 depending on model. Alternatively, you can buy a used one for cheaper but who knows how long that'll last.

 

I have had a driveshaft shop replace my stock u-joints with new, even though they're staked in. Not sure if all places do it but the one near me seemed to think it was perfectly fine.