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!