Solid axle swap and trade in value...

If you don't say anything about it, it won't affect the value at all. You take it in to the dealership, the used car manager drives it and that's usually it. He's just basically making sure it starts, runs and the wheels don't fall off of it. The odds are that even the tech that gives it a used car inspection won't catch it because most of them aren't "car guys". That said, if you decide to mention it, they might not even want to take the car. Most dealerships want nothing to do with modified cars, especially something as liability prone as a heavily modified suspension. I've worked in dealerships for years and this kind of thing happens all the time. My favorite was when I came in one Monday and saw a car on the lot that belonged to a guy I knew. It was a 94 V6 with GT badges and dual exhaust. They gave him GT value on his trade because they never bothered to open the hood or check the VIN. Once the papers are signed, unless the customer has lied about what the car is, the deal's done and the car becomes the dealership's problem.