I am assuming you have the 4L60E automatic. Does a Tahoe have a two piece or one piece driveshaft? Check the carrier bearing if two piece. Now define slack. Does it just seem like it is sliding into 2nd instead of a firm shift? If that is the case, you a in for a transmission freshening. It's better to take it in too early than too late. Take it to a tranny shop and have them run a diagnostics on the transmission.