Ford Ranger / B-Series :: 2005 - Squeaks At Low Speeds?

My ranger started to squeak at low speeds a few months ago (when it was pretty cold) and still does it in the mornings when I first start driving it. It squeaks from around 2mph up to 25-30 and then it seems to go away (or I can't hear it anymore cause of other noises haha). It is from underneath the truck somewhere, and not related to clutch or brakes. It seems to do it in the mornings only (or when it's cold) and increases in frequency when your speed increases. Not engine related at all.

My first thought was U joints or the driveshaft rubbing, since the squeaking was directly related to how fast you were moving and clutch in/out and brakes didn't change the noise at all. I couldn't find anything rubbing on the driveshaft.

Changed the U-joints since they were kinda old, but this didn't fix the issue. Next I pulled the rear brakes and checked all the springs and such, nothing I could see was wrong. Greased up the shoe contact points and put it all back together.

It was still squeaking, so I checked the wheel bearings for play and spun them while each corner was jacked up, the bearings seemed fine and didn't make any noise when I spun each tire. I have transmission and rear differential fluid.

I've checked all of the stuff I can think of and am kind of at a loss as to the next step. I guess the things it could possibly be are transmission bearings (input/output, since those spin all the time right?), pinion bearings or rear end bearings, wheel bearings (even though i've checked them), or ??

I've driven a chevy with bad bearings in the rear end, and this doesn't sound like that. The chevy sounded like a dull roar at speed, and kinda droned a bunch. this is a very distinct squeaking sound, very similar to a squeaking bearing on the idler pulley or something like that.