I'm well into work on my JD blower also - 58" front mount, converting to a three point rear mount. Avoid "front" and "rear" to describe the gearboxes, it could be confusing. One gearbox changes speed, PTO to fan speed, and will be bolted to the tractor side of the blower box. The other is a right angle drive, from the continuation of the fan drive, to the auger shaft.
I suggest that evaluate taking the whole auger assembly out, it shouldn't be that much work. Unbolt the auger shaft ends on the inside of the box ends, and the piece which suspends the right angle (Auger) gearbox from the box ceiling, and the whole thing should pull forward and out. There should be a spline shaft joint, which just slides apart from a universal joint in front of the fan. A spray of penetrating oil never hurts.
With that out, you'll have full access to the fan side. Yes, there will likely be a key between the fan hub, and the shaft from the PTO gearbox (on the tractor side of the box). That key may be held in place by a "grub" screw (allen key down a hole in the middle, no head). Again, spray of penetrating oil, and maybe warm up with a heat gun. Clean out the allen key hole, so the allen key will go well in. Don't ream out the allen key hole in the grub screw, the scope of the task will grow immensely!
With the grub screw out, the fan and hub should slide off the PTO gearbox shaft, and you'll see the key in there. Then you can remove the gearbox from the tractor side of the box.
All that said, for the work involved, I'd assure that you can get the needed replacement parts before you invest the effort. As said above, some parts are hard to get. The PTO gearbox may be rebuildable, though it'll be a machine shop type job.
With the PTO gearbox off, is anything (a shaft) obviously damaged/broken? Does it spin freely? If nothing seems wrong, before considering disassembly, remove the oil filler/drain plugs. Pour the oil out through a find sieve. Of no metal is in the oil which comes out and it spins freely, I'd refill the oil, and not take it apart at all.
If you get the PTO gearbox off, and need to take it apart, do it over a pail, it should be partly full of oil. At least two of the PTO gearbox bolts should be "shoulder bolts" (maybe allen head, not hex head). These serve to both hold the gearbox together, and to hold the two halves in perfect alignment. They may be a tight fit (that's the idea). If you have to tap them out, thread a sacrificial nut partly on, and hammer on the nut to get it started, don't hammer on the end of the male threads, you'd regret it later! If the two gearbox halves are stuck together, a sharp blow with a plastic mallet, or light tap with a hammer, on the side of the gearbox, not near a bolt hole should help - "pop" and oil comes out! Reassembly of the gearbox requires some special techniques, so don't get yourself there unless you really need to!