Contact Merchant Automotive, They have alot for the NP-1/Np 261 and NP-2/Np 263 cases, some may go into the NP 246.
Does your truck have the "Auto 4WD?
I am only familiar with the transfer cases in the 2500 HD's, but Eric at Merchant Auto will probably know where to get the good parts you need.
Here is thier website
Merchant Automotive :: Your Duramax Diesel and Allison Transmission Experts!
2006 1500 CC/SB with the 5.3L and Nelson PCM.
soon to come is a intake and exhaust. (wifes truck)
Other vehicle is a 03 2500HD CC/SB 6.6L DMAX straight piped and Edge stacked with Superchips and gauges, Loves to eat rice for sure! Fartcan exhaust makes it better!