You are right, and I apologize if I mislead you in any way. When I did my swap almost all the articles that I read stated to make sure to get a 96 and up that has disc brakes. Now, when I searched for a few minutes, I came across an article that states that all 91 and up Explorer shafts had 31 spline axles. Here are a few articles that I pulled up real quick.
One website says this:
In 1995 to 2000 trim, the Ford Explorer 8.8 rear diff is roughly the same width as the Dana 35 (3/4-1" narrower), has enormous tubes (3.25" diameter), an 8.8" ring gear, 31 spline axle shafts, factory disk brakes and come with 3.55/3.73 or 4.11 gears and sometimes a Trac-Lok diff. (Pre-95 Ford 8.8's and those found in Rangers and mustangs usually come with 28 spline shafts and drum brakes.)
http://members.cox.net/rough-riders/tech_page/ford8.8/ford8_8.html
Another website says this:
The '95 and newer were also factory equipped with 31 spline axle shafts. The pre-'95 8.8"s came equipped with a weaker (28)-spline axle shafts.
http://jeep.off-road.com/jeep/article/articleDetail.jsp?id=261419
A ranger website says this:
Most Rangers are equipped with the Ford 7.5-inch rear axle with several exceptions:
1) 1986-up "Incomplete vehicles" also known as "Chassis Cabs" were frequently equipped with 8.8" 28-spline axles, the '86-88 examples are distinctive as they are the only 8.8" Ranger rears that have no provisions for a RABS sensor.
2) 1990 & up 4.0-liter Rangers. These are ALWAYS equipped with the 8.8-inch 28-spline rear axle.
3) 1998-up 4x4 supercab Rangers regardless of powerplant are frequently equipped with 8.8" axles
4) FX4 Rangers 99-current come equipped with an 8.8 rear and 31-spline axles with either 4.10 or 4.56 axle ratios.
http://www.therangerstation.com/tech_library/Axles.html
Notice that some rangers came with 31 spline shafts, but it was only the 99 and up FX4 ones.
Good luck with the swap.