I have a 2006 gmc 1500 crew cab z-71. I am going to install a 6" lift this week with 20" xd rockstars and 325-60-20 nitto terra grappler a/t's. My question is about trailer pulling. How bad does this affect the ability to tow. I have a 29 ft travel trailer that i plan to pull to florida(700 miles) and i didnt know how the lift would affect it. I am also install a rear air bag set up. Also, what kind of fuel mileage decrease should i expect not pulling? thank you in advance for your help  

It will effect it REAL BAD!!!! It will struggle and you will be lucky if you do not fry tranny doing it. You need to regear with some 4.88's if you want this to work. You have another problem too. The large offset hitch you will have to use will place more stress on frame in rear and your frame is not that strong to begin with. Until 88, 1/2 ton and 3/4 tons used the same basic frame and with 88 thru 99 frame got weaker and in 99, weaker still. GM also reduced the leafs in rear to soften ride in 88 and further in 99. This mean more axle spring wrapup towing and with those tires it is even worse. To minimize this increase, you need more leafs in rear and mo spacer blocks in rear (this would be a bib no no towing with stock spring, and big tires too.) BTW, no chip or tune or exhaust or intake is going to fix gear ratio problem,with lift and towing, only new gears will.