I also have 4.56 gears in my front Dana30, so clearly that can be done. The only thing I can think of to explain what you were told has to do with the carrier. If you begin with a Dana30 that has 3.07 gears, most 4.56 ring and pinion sets won't fit on the stock carrier. However, there is an easy fix: buy a new carrier that does accommodate the 4.56 r&p.
If your Dana30 has 3.73 gears, no carrier replacement is required to upgrade to 4.56 gears.
My fleet: 91 Miata, 98 Wrangler Sport
, 01 Suburban 8.1L, 14 Impreza, 03 Protege
Jeep mods: 4" longarm + 1" BL, 35" KM2s, 4.56s with dual ARBs... and lots of scratched armor