Well, to be honest, the 3-speed is nice, but HPI diffs just don't hold up nearly as good as the ones in my aftershock. Even with all the parts upgraded to XL spec's and using alloy diff housings, they don't hold up like the stockers in my AS do.
Adding a lower 3rd gear to a savage just puts that much more stress on an already "weak" area. I have a 3-speed and it's cool to hear it shift, but honestly, getting it to shift where I want it is a pain since you have to take it apart to adjust the 1st-2nd shift (which I need to do in mine). I've upgraded my diffs in my savage completely and they are holding up ok now, but I'm fighting center axle cup issues now.
I've never had that issue on my AS ever. I've run many gallons through that thing without a diff issue. I did upgrade the diffs to HD spec over a year ago, but the ring/pinion/spider gears are all original and are over 2 years old with 14+ gallons of hard running on them.
The weak spot I can see on the LST XXL will be the chassis rails between the front and center skid. That's where it is longer than the LST2/AS trucks. I had busted mine at that spot 4 times prior to making an alloy skid to tie the front/center together. Haven't had a single issue since though.
As for the two speed in the LST XXL, you will find that you may want to gear it down 1T on the CB as it's geared pretty tall. The XXL has different trans gears in it that make it +1 to +2 over stock LST2's and it also runs a larger CB by 1T. I've heard vids of it and it really seems geared a hair too tall for the weight/tires and engine in that thing. Minor change to swap out the CB though.