What type of tire you plan on running? AT or MT? Brand?
Can be a 10+lb difference between the two and that rotating mass does add up.
For reference, I had a stock raptor on A/Ts (35s). Got mid to high 13's on my commute. Went to MT's, it dropped to high 12's, low 13's. And that was with a lighter MT. The toyos can weight over 70lbs per tire and some guys with Raptors get ~11 mpg with them.
I would expect the 3.21's to feel more sluggish going to the 35's, and it probably will downshift way more when you accelerate.