I'm changing trans fluid tomorrow. Its very dark like black - is it too late??- the trans was built 2 years ago.

 

does it smell burnt? if not it will be fine.

 

Not good for the trans. If it's black it's most likely burnt.

The "secret" of a long life for 4l60E's and most other auto trans is to keep the fluid bright red--not red, not darkish red, not brown, or, god forbid, black............

 

It don't sound good for the home team, d00d.

I'm thinking another rebuild is in your immediate future, no matter how short/long of a time ago the last rebuild was. That's pretty much irrelevant. Parts don't ask questions like that when they're in the middle of burning up.

What gear has been slipping? Which shift took forever? SURELY this isn't the first hint you've had that something wasn't happy??