I'm running the Rancho 4" suspension lift (it really lifts slightly over 5") which will fit 315/70/17 (~35") tires. You will definitely want and need a programmer for all of the reasons @
TimTom64b states above.
I did wait a few months to regear but it was definitely something I felt was needed. My truck was extremely slow taking off from traffic lights, carrying loads, going uphill, etc. and the transmission was running awfully warm at times during normal driving conditions.
Here is what I spent on basically the same set up you are considering:
*$1200 Rancho lift w/Rancho Shocks (installed myself)
*$300 for DiabloSport Predator (and if I remember correctly, $100 for a custom tune which was needed when I regeared)
*$800 (I think...anyway, it's very close), to regear to 4:56s (I had them professionally installed)
*$1200 for BFG A/T KOs 315/70/17s
I know some of the prices went up as I did this in late 2010 but this should give you a ball-park idea.
Also, I am not a mechanic but am familiar with vehicle maintenance and some repair, thus felt comfortable installing the lift kit myself after reading the instruction manual which can be found on Rancho's website. Something you may want to consider if think you can do and have 3 solid days to spare.
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Here is something that will help you visualize why the shop is recommending you change gears.