Which axle are you talking about?  Rear I assume?  And whether to lengthen
or shorten an arm to raise or lower the pinion angle depends on if you're
talking about upper or lower arms.  Which way do you need the pinion angle
to go?  Further up or down?

But usually, you should not even be messing around with the pinion angle
with the OE driveshaft, especially if you don't know what you're doing...
and also because the rear pinion angle should remain parallel to the t-case
output shaft's angle with the OE driveshaft still installed.

Finally, it's right at about the 3" lift height and taller where dropping
the t-case an inch is not enough, as you've figured out.  Instead of
attempting to adjust the rear axle's pinion angle which is not likely the
right approach in this circumstance, you'd probably be better off installing
1" taller motor mounts. Adding them is like dropping your t-case another
inch.  For a 3" suspension lift, a 1" t-case drop and a set of 1" taller
motor mounts like from M.O.R.E. is usually enough.

Jerry