its not weird, not only do different trucks suspension sag differently over time, people add weight to their vehicles, etc
BUT
All tires are different.
You ever measured the height of a BFG KO 31" tire? It's closer to 30" then 31".
Same holds true for all brands.
You should regear with 31's too, but you can get away without it.
People drive with 33's on 3.37 gears all the time, but yes the ideal situation would be to re-gear. You jsut get further and further away from your engines and transmissions powerband/gear ratio's when you dont re-gear.
If you plan to keep 31's then maybe do no body lift at all and just do a torsio twist and shackles.