Rear axle?!?!

I've got a 1990 B2200 with a 5 lug conversion. The trucks bagged and bodied and the 4 link is already all welded up to the this existing axle so I'm trying not too have to swap axles entirely but under any acceleration it is howling very loudly. I've never really messed with any rear axles short of changing out bearings and seals so I really don't know where to start.
When I got the truck all of the axle was already done so I have no idea if this is even a Mazda axle or a Toyota axle.
The pinion seal recently started leaking on me. Upon investigation I saw that there was no rear vent either. I guess in the previous owners efforts of cleaning up the axle and having it powder coated he shaved the vent smooth and never added a new one. So now I've added a vent and have a new pinion seal but I'm not sure it'll fit if this if it is a yota axle and if I need a new axle all together it may be a waste of time even installing the new seal.
This axle is quickly turning into quite a head scratching mess.
Any advice or ideas would be awesome.
Also I might add that this truck is my daily and aside from the fluid slinging out and the howling the axle doesn't pop or grind or anything and drives fine. I've put probably 1k miles on it like this while waiting on the new seal. I just check the fluid every other day and top it off when needed.
Thanks guy

Before you drop the hammer on the rear end, check your u-joints. If you have a two piece shaft, check your carrier bearing too.

Ill check them out before I trash the axle. I just can't imagine the U joints causing this horrid howling. Although I could see the carrier bearing being a possibility. In all actuality I guess it could just be a bearing also. Since this rear axle never got a vent installed the pinion seal has been leaking for a while I'm sure. I'm guessing somewhere a long the way it leaked out and the fluid level got way to low and hurt something internally. That's just a guess but it would make sense

My truck has a simmilar howling, it sounded like bad tire howl, and since I had winter tires on, I thought it was a possibility (although the tires did not howl that loud all winter), put my summer tires on, no luck. I noticed the howl dissapeared when I decelerated, or turned right, so I figured it was a bad right rear wheel bearing. Since my rear left axle seal and my pinion seal were leaking, I just replaced the axle, now the noise is still there. I have a center support bearing on order, hopefully that fixes the issue.

My howling started at about 20KM/h, was the loudest at about 40KM/h and was almost un-noticable at 70KM/h and above, but you could still hear it if you listened for it. At 40KM/h you could hear it over the radio (at a normal volume) and all the windows closed.

Mine seems to start quietly at take off and by 70mph or at close to any highway speed for that matter it is so loud you have to pull over to talk on the phone. It only does it under throttle which also makes me think its the axle. If I'm doing 70mph and put the trans in nuetral I can coast down the hill with it almost ompletely quiet but as soon as I start giving the truck power it starts again. It's really getting annoying to say the least, and don't even get me started on the oil slinging all over my back glass from the pinion seal lol.

I don't recall the Mazda rearends having a vent from the factory. At least I know mine doesn't...

They do have a vent, at least both of mine did, its on the top, to the right of center.

As for howling only when under load, that is probably the ring and pinion having wear. Could also possibly be the diff bearings.

I got the new pinion seal put in last night but I was unsure of how hard to torque the pinion back down. I think I got it about the same place as I marked it before I removed it. I'm pretty sure I'm just going to need a new ring and pinion. Mine is just getting louder and there's a lot if back and forth slop in it although the pinion bearing feel smooth. Maybe I can just find a new third member and put it in my axle housing??