[A1971Blazer]

If there's 1½" difference...I would think that it would be very obvious that the frame was bent....and easy to see that amount.....that's a huge difference.
But since you've measured the springs to be sure one is not installed backwards....that doesn't leave much else to cause the issue.
It also seems to me that if the frame is bent that much...the front end sheetmetal would be all out of whack?
Sorry..I can't offer more possibilities or advice, except to figure out what's going on before you put it back on the road...