I have a driveshaft that falls outside the standard “go to the scrapyard, and get another one” solution.

The slipjoint has a lot of play and shakes wickedly on deceleration.

*this is primarily a off road vehicle that sees limited road use*

What I’d like to try, is building up the surface with traditional bronze brazing rod, then grinding it down to restore a better fit in the joint.  

The question is, I don’t know what kind of heat treatments may be at play, and how brazing some material on would effect the hardness of the driving surfaces. ???????

‘85 Nissan720 if it matters.
It uses a weird arrangement of 4 half round grooves in the male and female portions, and then fills those (now circular) grooves with a series of balls/spacers.  

If you're putting bronze on it, why worry about the hardness of the material underneath?  Bronze is like butter.

ETA:  See if there is a driveshaft shop in your AO.  They can fix it.

The concern is altering the hardness of the grooves that the drive balls fit in.

I’d expect the bronze to be soft, but it’s purpose will only be to take up space and keep things (closer) to being aligned.

I just don’t want to wind up adding some material, and discovering 500 miles later that the entire driving surface has been degraded to mild steel butter.

Assuming that it was tight when new, the parts that wore and made it loose are the parts that need built back up.  They were steel to begin with, so you would be replacing the worn steel with bronze.