Tranny slips when warm, runs fine cold, why?

SOURCE: my 2001 gmc sierra 4x4

Fluid and filter change. You can do it pretty easily yourself. Hope this helps :)

SOURCE: I have a 2003 GMC Envoy, trans slipping that I have been towing

Low fluid can cause slipping. (depends upon how low) Most manufacturers post "safe" tow speeds. These are generally ignored. (not good). I recommend towing with driveshafts removed and where driveshaft has a "slip yoke" you install a yoke from a scrapyard and wire it to the tailshaft so it stays in place (keeps oil in and dirt out) Although you can't just detach the tow-bar and drive, it eliminates any possibility of trans damage, since the transmission cannot turn. (most use oil from pump to keep gearsets cool...when towing, pump is not pumping and over time and distance it can damage trans internals.)
Hope your problem is just low fluid and not a rebuild...keep in mind what I said though.

SOURCE: Put car in gear trans. Does not engage

tranny may need overhaul. It is possible that the vacuum actuator is leaking and will need to be replaced. Much cheaper then replacing tranny

SOURCE: My 2002 GMC Envoy transmission slips into neutral

try adding fluid and if still slipping neeed to have it checked could just need a fluid change

SOURCE: transmission slipping on 03 GMC Envoy. what does iy mean when trans fluid measure high on dip stick?

Two things: #1 someone has overfilled it by mistake thinking they were putting oil in the engine. Yes I have seen this.
#2 The transmission cooler in the radiator could be leaking antifreeze into the transmission through the cooling line raising the fluid level. I would look for milk shake foam on the dip stick and make sure you check trans in park and not neutral.