The "average" Ford 9" from back in the "glory" days had 28 spline axles and bearings equal/less than those found in the 7.5. Ford had no problem putting those axles behind 351w's in sleds that weighed a lot more than our Rangers.
And the 31 splines axles/Daytona pinion support/WAR cased 9" differentials have same axles*/bearings as an Explorer 8.8. And Ford had no problems putting those axles behind big blocks.
The 8.8 gets into trouble when you exceed 4.0:1 gears - with only 1.5" hypoid offset, you get into partial tooth engagement. The 9" with its 2-1/4" hypoid offset is much better suited for deep gears.
Of course, the aftermarket caters more to the 9". You can get stronger cases with bigger bearings that supports 35 spline axles, etc. I won't recommend the Mark Williams 35 spline axles for either 8.8 or OEM 9", the carrier walls get too thin - you start counting on the bearings to hold things together and that is cutting things too fine. But, Eaton will sell you a 33 spline differential for your 8.8, and Moser/Dutchman/etc will sell you matching axles, should you have concerns/pockets deep enough.
So, a moderate 351w with moderate gears (4.11 or higher), and street tires in a Ranger would probably be fine with a 7.5 - assuming judicious use of the skinny pedal; 8.8 should be fine.
*Ford 9" axles are cut with 45° splines, while 8.8" are cut with 30° splines, so would be about 20% stronger, but OEM 8.8 axles are manufactured from 1050 steel (need more carbon to be able to harden the bearing journals), while OEM 9" are manufactured from 1039 steel, which gets 15% back.