Hmm, I just took out 2 lst2 diffs in my lst2, 1515 2.5d on 6s with 23 pinion and 64 spur and stock lst2 tires. I purchased the truck as a used roller added all the brushless stuff and broke the two diffs, (tooth broke of pinion gears). The diffs were smooth when I received the truck, so I didn't open them up. My mistake I guess.
I've just fitted UE diff cases with ofna six spider buggy diffs to my Emaxx 3905
they fit up perfectly with no shimming (in my case) and seem very tough - I used to detroy maxx spider gears every few packs even with maximiser cups and flm combo bulks Neil has been using them with hot bodies diffs and I havent heard that he's had any problems Im running 6s with a 1600Kv 80mm Medusa and loving it - you will have to gear up or pick a higher kv motor like the castle 2200kv