The most common reasons for a gurgling toilet are clogged vent stack, drain pipe obstructions, and a septic tank issues. Since the drain pipe has been already rodded out I would definitely check the vent pipe and a septic tank condition (if you have a septic of course) before replacing the drain pipe.
Plumbing vents get clogged up occasionally by some rodents, birds, or even a beehive. If it happens, the problem would be most likely located in the upper portion of the vent stack / at the roof surface level.
However, since this problem occurs only when the upper floor toilet flushes, I would have the plumber check if the lower toilet’s connection to the vent stack is in good condition. Depending on the toilet’s distance from the main vent (and on your local code) there might be a separate vent pipe that runs from that lower toilet to the main vent stack.
I’m not sure what material has been used in your house and how old is it but galvanized and cast iron vent pipes tend to corrode heavily after a few decades. If that pipe is clogged (unfortunately it would be most likely hidden inside the wall) the lower toilet might gurgle if someone flushes the toilet above.
Infrared camera would help to determine if there’s a such vent pipe.