this is a copy and paste from a 4 wheeler mag tech article. Hope it helps
’98-to-present GM ½- and ¾-ton pickups, Tahoes, Suburbans, Yukons, Denalis, and Escalades
What you’ll find: NVG246
Type: Autotrac part-time case that always spins the front driveshaft, aluminum, chaindriven
Low range: 2.72:1
’99-to-present ¾- and 1-ton GM trucks
What you’ll find: NVG261
Type: Part-time, magnesium, chaindriven case
Low range: 2.72:1
’01-to-present ¾- and 1-ton GM trucks
What you’ll find: NVG263
Type: Part-time, magnesium, electric shift, chaindriven case
Low range: 2.72:1
’98-to-present GM ½- and ¾-ton pickups, Tahoes, Suburbans, Yukons, Denalis, and Escalades What you’ll find: NVG246 Type: Autotrac part-time case that always spins the front driveshaft, aluminum, chaindriven Low range: 2.72:1 ’99-to-present ¾- and 1-ton GM trucks What you’ll find: NVG261 Type: Part-time, magnesium, chaindriven case Low range: 2.72:1 ’01-to-present ¾- and 1-ton GM trucks What you’ll find: NVG263 Type: Part-time, magnesium, electric shift, chaindriven case Low range: 2.72:1 ’99-to-present Ford Super Duty and Excursion What you’ll find: NVG271 (manual shifter) NVG273 (electronic shifter) Type: Part-time, magnesium, chaindriven case Low range: 2.72:1