I pulled it out of the garage, cleaned up the previously removed rear drive shaft,
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I was hoping to replace the u-joints today but had more in store.
I found the collar of the drive shaft with a nice dent where the slip yoke is supposed to slide in.

(See 10 o'clock position in this pic)
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Truth be told, It fell off a few months ago as I reused the old drive shaft hardware and neglected to re re-torque it after my last u-joint change. I have new hardware for the next go at this.
The old hardware at the rear diff had worked its way loose, the drive shaft slipped off the slip yoke and hit the ground at low speed causing the dent.
Anyway, I heated up the collar and tapped a few times on either side of the dent and it opened up enough to insert the slip yoke freely.
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It's probably OK to run, after a balance, as a trail spare but I'm now on the hunt for a replacement rear drive shaft.
Does anyone have 1974 through 1980 FJ40 rear driveshaft to buy? It's a 60mm x 68.5mm flange to accept the 11mm bolts.