2002 avalanche 1500 transmission wont shift when it warms up

Hello I just joined the forum and had some questions. I have a 2002 avalanche 1500 with the My transmission shifts fine when you first start it up but after 10 to 20 mins of driving I lose all gears. I tried putting it in 1st 2nd and 3rd when it happens and it just revs up. If I wait 20mins it starts working again for alittle bit. Does anyone know where I can starting looking for the problem or will I need to have it rebuilt? the check engine light isnt on.

When you wait is engine off and you let it cool down?

Something inside Tranny is my guess and time for rebuild or rebuilt replacement.
Could be clutch or some stuck valve or broken part?

Fluid level

Yes I'll shut the engine off and wait about 20 mins and it shifts normal again until it warms up. The fluid level is good but the fluid itself is dark.

yeah dark fluid not good.
find a good local tranny shop and tell them your symptoms...