Because my driveshafts have 1310 yokes & that's what I'm using because I'm not wasting money on unnecessary stuff that I do t need. Another reason is I'd rather break a driveshaft U joint than grenade my Transfercase. I'm not running 44s, I'm running light 38s & I don't break things very often because I drive easy because I like wheeling & not getting towed to camp.
Anyway.. So Napa makes a 1330F to 1310 adapter U joint?
Are you sure???
Get a 1310 yoke and be done with it. I used to do the 1330-1310 thing but it's not worth it. I think RE has a 1310-1330 ujoint, but I'd switch to 1310-1310 and it makes trail repairs much easier.