05 Yukon Denali Speed Issues on 2005 GMC Yukon Denali

My 2005 Yukon Denali wont exceed 35mph. Why is this? It will go up to speeds as high as 45mph if shift into 4wd

Do you have any Service Engine Soon Lights or Service Vehicle Soon Lights on? If so, theses lights will mean that a computer code has been stored and this code can help you determine what the problem is. It sounds like your vehicle is in some kind of a 'limp home mode ' that is preventing it from going normal speeds.
2 more answers
Its sounds like you transmision is taking a dump on you. You shouls go get this checked out.
how do you shift into 4wd if its all wheel drive?