Like many others before you, you simply ran up against the error that is the chain auto parts store reference computer (and this is by no means the only error that has continued to fester unabated in their systems for decades
).
The 7.5" & Ranger 8.8" (thru '09) do use the same tubes, bearings, seals, end plates, and even axle shafts. They also have the same load capacities (2750lbs).
I often say to people: if you're replacing rear bearings, seals, or shafts on ANY Ranger regardless of engine or year (prior to '10), you always say you have the 7.5" axle. The only exception is if you have a '02 (only) FX4 or any year FX4-Level II Ranger (in which case for all parts except shafts, you say you have a '91-'94 Explorer).
Saying you have that year Explorer also avoids all the headache questions about your front axle when replacing brake or wheel-end parts on a '91-'94 4x4 Ranger too.
2010 and up Rangers got the larger tube 8.8" but axles stayed same size
I think the '10, '11 Ranger axles have the larger bearings & seals like the Ex 8.8, but are still 28 spline at the diff.
I recall there was a thread on this just a few months ago too, I'll see if I can find it...
Yeah here it is:
http://www.therangerstation.com/forums/showthread.php?t=166349
Definitely has bigger bearings (and they are not interchangeable with earlier shafts).