No, Jeff Dunham (born Jeffrey Dunham on April 18, 1962 in Dallas, Texas) has never appeared on the TV series America's Got Talent.
The man you are most likely thinking of is Terry Fator (born Terry Wayne Fator on June 10, 1965 in Dallas, Texas). He was the singing ventriloquist that won the America's Got Talent show in 2007 and credits Jeff Dunham as "a huge inspiration to me."
Tip! You're standing right in front of us, but your voice sounds like it's coming from over there! Color your friends impressed by learning the fine art of ventriloquism shared by Howcast.com.