Can you elaborate on this? I was about to pull the trigger on underdrive and low range gears recently but decided against it after I was told about the new input shaft and idler that put the cost over the price of diff gears. Wondered if an 80 t case could have compatible parts. I could rob the components off a $150 junkyard 80 t case and still do the tcase gears myself for less.Btw, with the older 100 series trucks, you don't have to do the input shaft and idler gear, so it's only about $1000 for the underdrive and 3:1:1 gears. That would make this decision easier, but unfortunately I need the extra $600 shaft and gear.
I have a 2000. Do I still need the input shaft and idler? If not I would do it in a heart beat.
FWIW i just put 35s on. Coming from stock size tires. It's got slower acceleration but I actually like the lower RPM on the highway. I felt the gearing was too low before. I would cruise at 75mph and be at nearly 3000 rpm. Now it's more like 2400 (but the speedo reads 68)
In your situation, you've already committed to installing lockers. Just do the gears with it. Your old gears are slowly wearing, increasing backlash and increasing the possibility for breakage especially as you start to wheel it with bigger tires.