allison tranny question

ill try to make this short. i have a 2001 sierra 2500hd with the automatic allison tranny. i was goin up a hill and it felt like it started to slip. got to the top and started going down the backside doing about 50 and the rpms where between 2200 and 2400. it wouldnt shift out of gear until i gave it alot of gas. turned around to go back to the shop and it seemed to be just fine. i changed both filters and fluid in tranny. plus no check engine light ever came on. today i noticed if your driving and hit the tow haul button it will not downshift at all until u hit the gas. is that normal? just curious if anyone else has experienced this. one more thing. when you apply the brake to slow down its like you can feel it shifting down in gears.