I just change my car battery in my gmc 203xl envoy but its running with less power and shake when i slow down what is wrong please can u tell me thanks

Clean your throttle body with throttle body cleaner, must be nice and shiny. Do this with the battery disconnected. When finished, hook the battery back up and drive around for 20 minutes or so, so the computer can relearn the newly cleaned TB. Your computer learned some bad habits with the dirty throttle body, so when you disconnected the battery, it thought the throttle body was clean, therefore, you'll get performance problems. Especially when the defrost or A/C is on.