Which transmission do I have?

Ok, I just inherited a 99 Tahoe.  It is the gas model (no diesel) and as I was driving it the service engine soon light came on.  I took it to the local AutoZone and they put their handy dandy computer on it.  It says that it is a downstream O2 sensor.  Now for my question.  I was trying to get a price on a sensor and the guy asks me if the transmission is a 60 or 80.  At this time I felt like a total Tahoe tard, I have no idea what he is talking about.  Is there a way to find out which I have without casting bones or consulting a medium?  I am brand new to the Chevy world, but I have changed my share of O2 sensors in different vehicles.  I understand the concept of upstream and downstream, I just have no idea which transmission I have or how to find out.  Please mail me at ronin352003@yahoo.com or ronin08@gmx.com with any ideas or websites that will help in my quest to spend more money.  Thanks in advance for any and all help.

Welcome to the Tahoe forum.
 
Have you asked a dealer how to tell which tranny you've got?
 
There's someone around here that can probably answer your question too.

You probably have the 4L60E the 4L80 is a heavy duty tranny the 60 is the equivilent to the turbo hydromatic350 the 80 the TH400...
Look on the side of the tranny sometimes it stamped right on the side. Your RPO codes in your glove box can also give you an indication.
If the truck is a 1/2 ton 2wd its probably the 4L60E

Drive by any tranny shop and they can probably tell you just from looking under the vehicle
 
if you have an RPO code of M30 its the 4L60E
if you have an RPO code of MN8 its the 4L80E

Here's a site that I've found useful.  http://www.drivetrain.com/autotranscrossref.html#CHEVROLET%20GMC%20%20&%20GEO