I'll pretty much echo Evan except to remind him that GM didn't put a Dana 60 in the rear of those trucks.
For sure you've got the GM Corp. 10 bolt in the front, but a Dana 60 out of a K30 will bolt right in when you need the added beef. If you've got the std. GVWR you'll have the 14 bolt 9.5", if it's a camper special with the heavier GVWR you'll get the 14 bolt 10.5" full floating rear axle.
Are you sure it's an NP203 transfercase? Stock was the NP208, an aluminum part-time chain driven t-case. The NP203 is a cast iron full-time chain driven case from 73-80, also it is longer and has a squat gear reduction box before the power dividing portion of the box.
Steering Box Brace
<- Clicky. The writeup they have there will explain why, it IS necessary, not always first, but inevitibly.
How is your 6" lift achieved? For the rear, I would recommend: Shackle Flip Kit
then 2" blocks at the most, otherwise you'll start to get axle wrap.
If you don't have the 14 bolt FF and plan to swap one in, at that time I would do this: Ubolt Reversal Kits
No, I don' work for ORD, they've just put a lot of time into fixing all the little things that need help on our trucks and they do make good stuff.
Sierra Classic 2500HD SLT, 4x4, EC/LB, LBZ/Allison
, Summit White with Charcoal Leather, dealer installed Westin Platinum Series bars and a Line-X.
R.I.P. Jason (willyswanter) Payne