Go-To guy for tranny in Phoenix

2007 5.9 auto, I recently started pulling a 12K pound fifth wheel. Getting a very harsh 2-3 up shift, I have to watch the speedo and start feathering the throttle at about 30mph, then I get an easy shift but it makes it a challenge getting the rig up to speed from a stop since I can't lay into it. I have 85K miles and zero mods and never had any trans service or maintenance done. I think I'm due. Looking for recommendations in Mesa, Gilbert or surrounding area.