This is what I did to mine. It is not perfect as each vehicle will vary some.

4wd to AWD conversion how to.

Step 1. Locate parts.
A. Parts needed.
1. Front differential + new fluid might as well change it now.
Part Number:?????????
2. Front Drive Shaft from Silverado SS or Denali or Escalade
Part number:15186002
Rear drive shaft will work if you have an original GM yoke depending on your application. If you have vibration you need a longer shaft.
3. AWD Transfer case. Transfer case Fluid + transmission fluid.
Part Number:NVG149 / Fluid part number 88861800
4. Cup to replace 4wd switches in dash.
Part Number:?????????
5. Zip ties.

Step 2. R&R front Diff. Leave front Drive shaft out till transfer case changed. Locate front diff actuator wires and plug. Tape up plug and pull it out of the wire loom and zip tie it out of the way some where. You could cut it but I did not cut mine and don't know what would happen with the pcm or codes.
Step 3. R&R Transfer case.
Step 4. Locate Speed sensor connector that plugged into Autotrac transfer case. There will be three I think. Only one will plug into the awd Transfer case. Zip tie the others out of the way. You may want to tape up the plugs to keep water out.
Step 5. Put driveshafts back in. Make sure you put the SS shaft in not the old one. I reused my rear shaft. I am not sure if this will work for everyone because my shaft was custom made.
Step 6. Locate and unplug the Transfer Case control Module or TCCM. Unplug and remove the 4wd switches. Replace the 4wd switches with cup. Unplugging the TCCM eliminated any trouble codes for me. I am not sure if it will for you.

Pics of the differences in Differentials.

The picture below is what the vent looks like on awd front diff. A technical bulletin for cold weather changes this. Info can be found at the link to Denali forums. If you get a diff with the white tube do not worry about the bulletin as it will not apply to your diff.

Pics of different drive shafts.

Side notes:
Simply unplugging front actuator on 4wd diff wont work. It will just free spin. This will kill the awd transfer case. It might be possible to shift it into 4wd then unplug the actuator. I am not certain of this. It is worth a try. I did not try it because I changed gear ratios during the swap.

AWD /Autotrac / Manual 4wd
The awd transfer case is a 62/38 split I think. It uses a viscous coupling to be awd. It is not selectable.
The Autotrac transfer case is a selectable unit. Auto 4wd/ 4 Hi and 4 Low. Auto 4wd will engage 4wd when the rear wheels slip. In my truck driving on dry pavement caused loud growling and once the rear wheels lost traction the front wheels would violantly grab and loose traction. Awd is smoother, quieter and less violent.
Manual 4wd has 2 wheel drive mode and 4 hi and 4 low.

Pros of AWD transfer case.
Traction all the time.
Better tire wear.
Safety in inclement weather.

Cons of AWD
I have heard of a few breaking.
Not selectable.
No smoky burnouts.
No 4 Low.