2004 Yukon XL occasional slow 1-2 shifting

['63GENIII]

Hey guys, I'm usually on the 60-66 side but I'm servicing our '04 Yukon XL (Denali with the 6.0) for a last minute trip to San Diego for my daughters softball next week. The question is with the transmission which has had an occasional 1-2 slip when warmed up. It has done it for the last 40K and we have just lived with it, meaning when it does it once, we baby it through a few cycles of the gear shifts and it seems to go away. So far so good right? Well I was thinking about changing the transmission fluid this weekend and am wondering if there is something that I may possibly be able to repair while I'm in there or am I possibly about to open a can of worms by doing this service a week before a long trip like this in the first place? I really don't want to take it to a transmission shop because I know they will tell me that I DO need a new transmission ( I actually took it to one when we first got the car and that's what they said). I kinda trust the opinions of the members of the forum a little more than alot of the shops that I've found. Anyway, is there something that I should be looking at - for when dropping the pan, or is it a bad idea to even open it in the first place before a long trip when I don't have the money to replace the trans in the first place? I appologize for the novel, just trying to get an opinion from a well respected source.