28-spline is fine unless you plan on carrying large loads in the bed or are dropping is a 250+ HP engine

What you should look at is the Ratio of each differential, to see if they are the same

They are swappable, plug and play, the mounts will all line up.

8.8 weighs more, not much but with a 4cyl engine MPG might be more important than strength, your choice.

You can get axle ratio from drivers door label, have a look here: Ford 7.5 & 8.8 Inch Axle Tag & Door Codes

Switching from a OPEN axle to a Limited Slip(L/S) axle is always a good upgrade
Many 4x4s came with L/S in the rear axle

Ratio of rear axle matters for performance of engine
3.73 was most common, reasonable MPG and reasonable load hauling
3.45 was better for highway driving, lowers RPMs at 65mph, but less pulling power at low speeds
4.10 was better at low speed and pulling, but higher RPMs at 65mph so not as good with MPG

1997 Rangers still had the speedometer hook up on the transmission tailshaft so you can change the Driven gear to match axle ratios and tire sizes to correct speedometer to true speed

 

Yes, the one in my truck right now is a 3:73 and the other is a 4:10 i'm not too concerned with fuel economy, i just want a stronger rear end, plus its free cause i already got it! another question is i was looking at my 7.5 and there is no sway bar but the 8.8 has one, do i need to swap this over too? also thanks for the info on the speedo hookup ill have to do some research on that.

 

You will like the 4.10, it will make truck feel "peppy"

On trans tailshaft drivers side you will see the VSS(vehicle speed sensor), 2 wires hooked up to it.

It is held in place by 1 bolt

You will have whats seen in pictures 3 and 4 here: Speedometers ? How They Work : The Ranger Station

The black gear in picture 4 just slides off and can be replaced with another gear with more or less teeth
And thats all there is to re-calibrating speedometer when you change tire size or axle ratio..............on 1997 and earlier Rangers, 1998 and up get way more complicated, lol.

 

Could always go for the expo 31 spline. You get disc brakes and the 31 spline. I'm about to put carbon fiber clutches in mine at some point. Might wait until the posi in this one goes out. We'll see. The expo isn't terribly difficult but does take a little time to get together all in all a great upgrade from my 8.8. I do wish I had 4.10's though they are harder to come by.