You will be hard pressed to find 4.56's in an axle for a price worth paying. Keep in mind that a used axle with4.56's is not stock. So someone put those 4.56's in there for a reason most likely that lead to them being abused in one way or another.
Now to install your own gears take a lot of time and very precise tools. It is not a job that you can just try to get right. Gears are not cheap and a mistake could lead you to buying them twice. If you want 4.56's you will have to buy the ring and pinion and have them installed. If you want 4.10's you can find a donor vehicle from the junk yard and take its axle's. Installing whole axle's is easier than installing gears.
With that being said. If you have a 4.0L engine then 4.10's would be enough. Especially if you have a stick. If you have an auto 4.10's can even be enough. If you in the future hope to go to 35's then 4.56's would be the better bet. If you have 4x4 you have to change the gears in the front and the back.
Now that is alot of IF's. So to better give you an answer give some information. What engine, what gears currently, auto or manual, what kind of tires, where you drive the truck(off road highway city). All these things are needed in answering you question properly.
It would be best to put some information in you signature. Had you had info there I could have given you a better answer.
98 Xlt 4x4 auto 4.0L
PA Body lift, 33'' Super Swampers TSL's.
Chevy spring swap with torsion crank.
Explorer rear, 4wheel discs, 4.56, and a rear lunchbox locker.
K&N drop in fliter and cowl box snorkel.
Herculined bed and a topper.
Diamond plate skid plate.
Herculined interior floor.
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