Torque Tube measurements and differences, I think originating from Erkka Vilhuen.

TT dimensions:

25mm is the first version, also known as the constant diameter drive shafts, used up until sometime in 1987 for the automatic TTs but were the same for all years in the 5 speed TTs.

28mm is the later version used in automatic TTs.

Is there any way to tell by inspection if I have the 25mm shaft without taking the torque tube out??
If it is a 87 auto and it has not been changed, it should be a 28MM shaft.
You can check on the drive shaft by removing the front flex plate cover and measure the drive shaft that way. The drive shaft will be at it's true diameter right before it enters into the TT. The 28mm drive shaft has a pretty noticeable step down to the front splines, while the 25mm has a small, turned, step down.