Did you crank the torsion bars for the front lift or no? If you have and have not done an aligment, that could be the reason for chhirping tires, possibly.

Other than that, mileage might vary depending on many factors, I doubt that it was the shackles alone. If the weather got cooler, your mileage may decrease a little. I always see my mileage go down little by little as it gets colder in the season.