Welcome to the Forum from NH.
Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.
I hope that you will become a participating member in the Forum's discussions.
Pics, please.
I cannot 'positively' confirm this for you based on personal experience, but I used 6.5" speakers in the doors of my 1997 Yukon SLE and my 2007 Yukon XL Denali. So, it would make sense to me that the 6.5" speaker would fit in the 2000 Yukon SLT doors.
According to Crutchfield, a great site to shop for audio system options, even if you do not eventually purchase from them, a 6.5" speaker will fit in all 4 doors for your 2000 Yukon SLT.