[A1971Blazer]

OK without seeing it in person or pictures, I'll throw out something else to check.

Check the springs to see if they are the same distance from the spring eye(s) on the end to the center pin on the springs. The wheel would be off center in the fender opening if they were somehow backward?

Are the replacement springs arched upward? The factory ones are and when the spring compresses, the bottom of the rear shackle will move forward.

It just doesn't make sense that the driveshaft would be too long if the axle hasn't moved rearward. Did you have the new driveshaft in place before you replaced the springs?
Are you sure that the driveshaft is compressing properly on the slip joint?

Not trying to question your judgment or ability, just throwing out ideas

The door gaps will change when you jack it up by the frame...that's normal, the frames are really flimsy on these