Machinist help and/or information, please

Maybe figure out a way to adapt a ready-made steel lead screw and nut. This is a 1.5" dia. x 6' screw that should have the capacity: https://www.mcmaster.com/98935a331
Only $100 and simplistically speaking, perhaps set the existing part on top of the nut to make the connection to the mating yoke. There are a variety of nuts, including bearing bronze, to fit. The bearing bronze is a few hundred $$, but cheaper than machining a new one, I'm sure.

There are also ball bearing lead screws available, but I didn't research them.

 

This is an excellent idea. Acme threads are plentiful. If you change one rod/nut pair just make sure they have the same pitch so it advances at the same rate as the others. Also for acme threads check they are OK to use if the rotation speed and load is within their spec limits. It's probably fine.

Occasionally any linear screw needs cleaned and lubricated. Especially if they are not enclosed and subject to dust. If the one nut is worn out the other three may be soon to fail.

 

Was wondering if the part in question has a part number cast into it that might be googled.

 

Acme threads won't take as much unidirectional load as an equivalently-sized buttress thread in its strong direction. Not saying you wouldn't be able to make it work and be safe, but that'd be something to carefully consider when replacing the part.