2000 Blazer transmission issues...still

Spent the 3K to get a new tranny put in my 2000 blazer. It was a GM rebuilt one 3year warranty deal. When the engine is cold, It doesn't want to shift out of first? Or at least that's what it sounds like. Then engine revs but the truck seems to be stuck in first and won't shift to second. After a 2-4 second period, it will finally shift and then the problem seems to go away. Although my wife, who drives this truck regularly said she noticed it when going around 40mph too. It seemed like the truck needs to shift, but won't and for a few seconds your kind of stuck at one speed before it finally shifts and you begin accelerating again? Whats up?