Differential Question
 

Before I return the 1984 300TD Model 123 (Orient Red) to the shop to check if the correct Diff. was installed shortly after I got the car, I'd like to know if members with the same model have a similar performance?

Consistently about 20.5 mpg, mostly highway driving.
40mph @ 2800 rpm
50mph @ 2300 rpm
60mph @ 2700 rpm
67mph @ 3000 rpm
80mph @ 3600 rpm

If I exelerate (pedal to metal) from standstill making a curve, like in an intersection, unloaded rear tire would give me a short wheel squeel when the second gear engages.

Is this normal?

I know the numbers you posted for the 3 highest speeds are very close to the ones observed in my '83 300D. I've been getting 21-24 MPG.

mine does about the same speed at those rpm's. your mileage seems way too low for highway driving imo.
my car used ot give a chirp going into 2nd as you describe but the tranny wasn't getting enough vacuum. turning up the vaccuum a bit made that stop, and hopefully resulted in a little less stress on the diff.