Traditional currie 9's used 8.8 gears. that is why "Truhi 9" came to be.
609 just refers to any blended combo of d60 and 9" components, but the most basic concept is any 35 spline 9" center section is the starting point for a 609 build.
The fronts are like Vince said, most commonly a 9" center with 60 outers.
There are literally dozens of configurations available with prices that range from 2500$ to 15000$.
Lower the cost, more junkyard and labor involved.
The most affordable, reasonably buildable rear 609 is a standard truhi 3rd member, spooled, or Detroit, fabricated housing of choice, and a currie 35 spline semi float axle kit. I built one of those with a friend years ago for under 2500$, but pricing one today I would think it would come in around 4k.
Front 609's is where it gets incredibly expensive unless you use a lot of junk yard parts. Finding someone to cut off their ford inner knuckles from a front 60 to put on a fabricated housing may be a challenge, but Im sure it can be done.
The little currie center, fabbed housing, custom shafts to use all jeep outer stuff is a decent little axle, but more suited to a stock 4 litre and 35's.
I've built a few for myself, and had a hand in building several custom 609's. If you are ever wanting to shoot the crap and talk axle talk give me a shout. The coffee's always on.
Oh ya. My rear 609 was a 35 spline full float Truhi 9 dual laod bolt, Detroit center with 538's. Superior shafts and rock crusher 60 spindles and hub kit. Custom brakes using wilwood aluminum chev calipers and chev rotors.
Entire setup came in under 300 lbs. About the weight of an 8.8 and had nearly two inches of clearance versus a shaved 14b.
My front used the same center section, spydertrax custom knuckles (45 degree steering), F450 bearings, chev brakes, PSC full hydro, spydertrax chromo shafts and yokes, put together with d60 ctm's. That front diff came under 400lbs. A rock crusher 60 crate differential comes in at nearly 700lbs for comparison sakes. Junkyard 60's about 550lbs before you load em up with stuff.
I finished that rig 10 years ago. Beat the squirt out of it with dual tcases, a mean stroker, 38's and a heavy foot for 4 years before I sold it.
Its had several owners since, and as far as I know, nothing has failed in those diffs.
Start to finish, all the work myself, I spent 10k building those two diffs 10 years ago.
Today, to go all out and do the same, you would probably be just a hair over 20k.