Could it happen & when was the last time.

[Lolmorgan]

I've just found the answer.....

The last time Sheffield United, Sheffield Wednesday, Doncaster Rovers, Barnsley, and Rotherham United were all in the same league division was during the 1979-80 season.
During this season, all five South Yorkshire clubs competed in the Third Division (which was the third tier of English football at the time). Barnsley, Sheffield United, and Rotherham were all close in the mid-table standings, while Sheffield Wednesday won promotion at the end of the season.

Doncaster was in the 4th Division.