the shackles are 5" or 5.5". not exactly sure. as long as you get one in that ballpark youll be fine.
a well blended toyota rear pack will clear 37's as well (with some fender trimmage). you just have to build it. that being said, i prefer the aftermarket springs because they're fresh, they flex well, and they dont squeek. not to mention, the installation is easy because the length of the spring puts the fram tube right between the body mount. i think the tg springs are $250 a pair. thats what i'm putting on my rig. :awesomework:
the 5" spring might leave you a little bummed at first because theyre pretty tall. it depends on what you want though. some people like the extra inches. if not, you gotta let them break in and sag a little before your rig will start to crouch. either way, i think the 5" is too tall of a spring regardless of the tire you choose. i plan on running 40's for my build on 4" springs with 2 leafs removed so it sits even lower. to each, his own. good luck. :beer: :awesomework: