You are right Super, it was diff Y, diff X - had to be to rhyme. Anyway I believe they were still doing it in 82 when I graduated
"Diff Y, Diff X" came after the original "Diff X, Diff Y" cheer. The original went ---
Differential X, Differential Y,
A-squared, B-squared, Integral of Pi.
Engineers Touchdown; Engineers Yell.
Georgia Tech Yellow Jackets, FIGHT LIKE HELL!!
The revised one went ---
Differential Y, Differential X.
To Hell with Differentials;
We Want Sex.
The revised one was funny, but you just can't beat the original one for, well, originality.