Grinding and grumbling noise when slowing down

Hey, everybody, hope you're all doing well!

Here's my info to start out with--'01 PreRunner Xtra cab, 3RZ-FE (2.7l).
This past Saturday on my way to work my truck started making a loud grinding/grumbling sound accompanied with a bad vibration. This started when I began to slow down for my first stop sign, from about 50mph. I thought it was my brakes, since braking seemed to be what induced it. 2nd & 3rd stop signs, same thing, though each was from a progressively slower top speed. It seemed like it was coming from somewhere behind/below the cab doors and after tense time of deceleration from freeway speed I realized that my brakes still felt like they were fully functioning. Front brakes were replaced last year but the rear drums haven't been done in a long time.

On my way home later that night the issue was still there but by the end of my commute the sound/vibration had started occurring not only when braking but also when I just let off the gas pedal. As soon as I gave it more power it would cease.

I started wondering about my transmission. I've had a leak in the trans pan gasket and just to make sure I still had fluid in there I checked the dipstick. Sure enough, it read as being full. It hasn't been changed in a very long time and though it looks decent I'm going to change it on my next day off. So, lack of transmission fluid isn't the culprit.

Then the non-greasable u-joints in my driveshaft came to mind. I've had issues with those squeaking and barking (also affected by changes in throttle input) and spraying the mating surfaces once annually with white lithium grease had taken care of the problem. If one of them has ultimately started to seize up, what exactly would some symptoms be?

This truck is my only vehicle and it's my baby. Thanks in advance for any bit of help you can offer!

 

The first thing I would do is jack it up and roll the wheels to check for bad bearings. Listen and investigate anything that doesn't seem right. Then pull your wheels to check out your breaks again. Just because you changed them out doesn't mean that they are not coming apart or something has broken and rolling around in there getting hung up in different places, making the problem seem to have different symptoms. The u joins usually start making a knocking noise when going out but I have heard of them giving bad vibrations as well. That being said, they are an easy and cheap fix if they are old and you want to eliminate them. That is all of the limited knowledge that I have, good luck finding it.

 

Turns out my driveshaft center support bearing is shot. It's got a bunch of play in it, allowing the shaft to flop all over the place. Just ordered a replacement. That'll solve at least part of the problem. I also heavily regreased my u-joints and will check out how bad my wheel bearings are (front left has been in need of replacement anyway) and deal with them accordingly, next chance I get. I plan on replacing my beat-to-heck RaceRunner coilovers soon (Aside from previous owner's lack of care for them, I'm pretty sure they're beyond rebuild-ability due to the corrosion they've sustained living through WI winters without being cleaned enough), and will get new tires/alignment when that is done. Hopefully in the next month or so, all will be well.

 

Well I feel like an idiot. Blown axle seals and low-to-no differential oil. Thankfully a friend of mine is a mechanic and realized what was going when I had him drive it for a few miles.