A couple wheel/suspension quirks I'd like to run by you...
 

1992 LS-L, mostly stock, as far as I can tell.

At around 65, the tires [and steering wheel] start vibrating like hell. Once up to around 75, it smooths back out. I'd assume this is just a rotate and balance issue, no?

There's also a very high-pitched squealing/grinding noise coming from the rear. I'm guessing this has something to do with the suspension, as it seems to come from the outside during sharp turns, and from both rear wheels during hard acceleration. I figure the extra weight being shifted is pushing something and causing the noise. Any ideas? I thought it might be tires rubbing, at first, but that's usually a much lower-pitched sound, in my experience. They're not original wheels. As far as I can tell, they're the 17x8 Enkeis off the Lancer Evo VIII. I believe it's a 40mm offset, and the tires are 235/45s, giving a nearly identical outside diameter to stock...

Couple of things:
1. check the bore diameter of the rims. The SVX is a "hub-centric" vehicle, so it depends on the hub to positively locate the rims, not the lug nuts. You might need some adapter rings to line things up right
2. Definitely have the front tires rebalanced.
3. Check the shield that goes behind the rear rotors. You may have bent it slightly so it is rubbing against the rotor on hard turns
-Bill

It seems like like a tire rubbing the fender could explain the noise. I guess you've looked for rubbing marks on the tire/fender?

Re: A couple wheel/suspension quirks I'd like to run by you...
 

Actually...
a 40mm offset will make the tires stick out more than stock. Also 235 series tires are also wider than stock.

Combined, your tires would be sticking out almost an inch further.

A 235/45-16 tire would be a little wider and a little shorter than the stock 225/50-16.

Mine used to do that, vibrate slightly above 70, a lot above 80 but smoothed out above 90. Resolved completely when I got new tires (my tires were old enough that I didn't bother getting the alignment done to try an fix it). :)

Thanks, I'll check that.

And Earthworm, these are 235/45-17s... less sidewall, yes, but more rim.

Ah, yes, I missed that one
:o

Mathematically these tires come out to 25.33" tall compared to the stock 24.86" as a result your speedometer will be fast by only 1.89%