trans ?

I have a 99 2wd sierra with a 4l60 in it and I was changing the oil in it and decided to check the tranny fluid level and it was fine but the fluid was noticeably darker than it should be but it doesnt smell burnt or anything the tranny itself shifts fine and doesnt slip should I just drain it change the filter and fill it back up with fresh atf and go or should I be lookin into gettin it rebuilt

Pull the pan and look for debris, that will tell you what's really going on.

Is there a drain plug in the pan on the 4l60's or do I have to drop the pan to drain it ?

Some do and some don't, you'll have to look and see.

regardless pan has to come off to change filter.

Sure sign to start Savin penny's for a trans. While you have ur pan off look n see if ur valve body has been changed. Will be the last 2 numbers of the year. You want to have an 01 ur up

Dark color is clutches trust me I'm in the middle of a rebuild