So I was wondering why when I've got the car in 3rd or 4th/OD (OD might not be it...whatever it is) and I shift down to 2nd it sounds like my wheels are spinning and burning out? Never had this happen on my newer cars. As far as I know I've got a 700R4 with unknown gears and an LG4 305.

Dude that's actually COOL lol. But an lg4 shouldn't be capable of that lol. Stock..

Man, I have no idea what I've got in the engine, lol. I just know it was a 2.8 swapped to a 305 and the parts manager and I at my last job got the engine code and found out it was an '87 305 roller motor most likely out of a Monte Carlo. This is my third quadrajet on the car it it's the best so far. I've done nothing to it other than new carbs and new plugs (and fluids).... I've got **** stacked up to thow on it it at some point.......but this has been a mystery to me. What causes this?

I'm not a tranny expert so I don't know specifics, but I know it's transmission related. It's shifting hard or not meeting up right or something along those lines, and giving you a little squeal as it figures out what it's doing. Locks up or travels slower than it should momentarily.

I'd try to get it fixed if I were you. Something like that can't be good for your tranny guts, nevermind tire wear.

It's only when I downshift manually and it's an auto tranny. When let the tranny do it's thing and downshift by itself it seems like the car idles really low like it's doing to die or it's not getting enough gas and then it gets back in sync and is fine.

In addition to something being wrong either with your trans, or the way you downshift, you should take a look at your suspension. I had a hell of a hard time getting traction with my car until I replaced my shocks and struts.