what axles do you have on the 06 mega cab?
FOR THE REARS DEPENDING ON WHICH AXLE YOU HAVE
the 9.25 requires 75w-140
the 9.25 LSD requires 75w-140 with 5 oz additive LSD
the 10.5 requires 75w-90
the 10.5 LSD requires 75w-90 as well. with NO ADDITIVE

FOR THE FRONT AXLES
all front axles require 75w-90

 

Looks like 2.5 liters or 2.63qts of 75w-90 fluid.
2006 specs HERE