How many slines? 28 or 31?

Yes, yes, and maybe. In that order, I think.

Anyway, yes your axles are 28 spline, the explorer (and post '86 F150) is 31 spline, and the housing and axle need to be of the same spline count to work.

You could change the side gears on the 31 spline to 28 spline, but this is a little bit of work, and you would need to get 28 spline traction-lock side gears.
Or, you could try to find 31 spline axles, but that is a bigger task. Best to sell the 31 spline as it is worth a few bucks more than a 28 spline, then go back and get the correct one.

Well a good 31 spine traction-lock can sell for $150 on Ebay easy. Finding a 28 spline 8.8 traction lock is a lot harder than finding a 31 spline, if that makes any sence! 28 Spline t-lock's were fairly limited to the Mustang 5.0, and a few other cars like 87-88 T-bird turbo Coupe's/ Murcury XR-7 Turbo's as standard equipment, but were optional on the 5.0's T-birds and XR-7 of the same years. Also they were on a very few '86 F150's, but most 8.8's in trucks are open (non-traction lock) diff's. There are the occational Crown vic's and Town carss with them and of course many LSC's with the handling package had them (springs instead of air bag suspension is a good sign), and all Bill Blass models had them.

But 31 splines axles come in all Explorers, Bronco's after '86, and F150's after '86, some were t-lock, some were not, but all are 31 spline.

The 31 spline is a desirable part for upgrading a 4x4 and for people that want to go with larger diameter 31 spline axles in thier Mustang, so they go for a bit more.

You may find someone willing to trade a 28 spline for a 31, I would and have a 28 spine in my garage right now, but I don't think shipping it would be cheap.