Need to augment that info for you to fully understand what you have. Below is from a word document i created a while back.
FULL TIME AWD
NP3 (NV 149, BW 4473 viscous clutch): uses center viscous clutch to give front/back torque bias.
NV-149: 01-07 classic Sierra Denali, 03-05.5 Silverado SS, 02-older Yukon Denali and Escalade
BW 4473: 02+ Savana/Express AWD Vans... problematic, terrible design. DO NOT USE
NR3 (BW 4476, 4481, 4485, open differential): uses open center (planetary) differential with traction control (wheel-brakes) to transfer power.
BW 4476: 03/04+ Cadillac SRX
BW 4481: 03-06 Yukon Denali and Escalade
BW 4485: 07-current Escalde, Sierra/Yukon Denali
NR4/NR6 (BW 4493, 4494 open differential): Used in Hummer H3.
BW 4472 used in S-series stuff: Astro/safari, typhoon, syclone
FULL TIME 4WD
NR4 (BW 4482): AWD transfer case with the 4LO option. Has open center differential, uses traction control (wheel brakes) to transfer power to front wheels. Center diff locks when in 4LO.
Full size SUVs, H2
NR4 - Transfer case, 2.64:1, 2-speed, button-actuated full-time 4WD
JL4 - StabiliTrak, stability control system
RPO NR4 - AWD with 4WD Low
RPO NR3 - AWD with out 4WD Low (Denali)
RPO NP8 - Auto 4WD, 4WD, 2WD, 4WD Low
NR4 and NR3 use the same software algorithm to detect rear wheel slipage and put power to the front wheels, typically with in 300 ms.
Yukons come standard with limited slip rear unless the vehicle is equiped with Stabilitrac (begining in MY05, you can have limited slip with Stabilitrac).
Denalis come standard with Stabilitrac thus removing the limited slip rear and Stabilitrac is a option on the Yukons.
If your vehicle has Stabilitrak (RPO JL4) then you have the same drive system as in a Denali with the added 4WD low feature
PART TIME 4WD
NP1 (NV 233, 243, 263, and T-150 push button)
NP2 (NV 231, 241,261, BW 4401, 4470 shift lever).
Both are 2HI/N/4HI/4LO
AUTOMATIC TRANSFER CASE
NP8 (NVG 226, 236, 246, 246 EAU): 5-position t-case 2hi/A4WD/4hi/4lo/N. The A4wd is demand-based... locks center clutch pack to send power to front wheels when rear traction lost.
Also, to understand the New Venture gear nomenclature, study this:
Example = NV-149
1st two digits
NV = New Venture Gear
1st number
Number of speeds/gears (1= high range only, 2 = high and low ranges
2nd number
Strength (range from 1 to 7, don't have the torque ranges handy... in this example 4 means middle of the design envelope of NVG designs. 1 is weakest, 7 is strongest)
3rd number
Type of transfer case
1= part time
2= full time all wheel drive
3= electronic shift
5= torsen differential
6= computer controlled wet clutch
7= GeroDisc
9= viscous coupling
there is no 4 or 8, don't know why.
So you see that the NV-149 is a single range, medium strength, viscous coupling design (doesn't need the wheel brakes to transfer torque to other axle)
