2500, plain automotive style rear end (press in single outer bearing, hub part of axle shaft), like a car or a half ton.
3500, full float axle shaft, hub on spindle type rear end (like a real truck).
2500, and 3500 rear axles share no parts.
To keep the weight near the centerline of the hub you would only gain the width of half of one of the current tires, that amount however, times 2 (each side) is almost enough, especially if you went with a narrow tire like a 235/85/16.
With enough money, anything is do-able!
Personally, I'd rather try to find a way to make a Ford or GM rear axle fit. Lots of challenges there too!!