In the older suburbans there was an option for the 8.1 engine that upped the towing capacity. 2006 was the last year for the larger engine option. Here are the numbers I found from edmunds.com for the 3.4 ton suburbans
2006 - 9900# or 12000# (6.0 vs 8.1 engines)
2007 - 9700#
2008 -...