On most toyota transmissions the weakest gear is 5th.  4th is a 1:1 so it will be strong enough to handle higher torque but as a rule of thumb for me I try to stay in 1st 2nd or 3rd.  Most who run dual cases like you run in 2nd and 3rd.  1st if both cases are in low.  Its how I run mine with 4.7s in the rear.