Any increase in diff gearing helps. You would definitely feel the difference with 5.38 gears... IF you could locate a set. Very rare do we see them on the "for sale" list. I don't recall seeing any of these within the last year or so. They were only available on the 1991 4-door Sidekick with the 5 speed transmission. When they did come up for sale, typically the asking price for them was in the $500-$600 range.
The only vendor I am aware of that sells some much deeper gear sets for our trucks today is a company called 4Xfourart which is located in Turkey.
http://4xfourart.com/Product/77/VITARA-585-RING---PINION-GEARS-FRONT.aspxHe offers new gears in both the 5.72 and 5.85 ratios. The cost to purchase and get a pair of gears (front and back) from here to the United States is right around $1,000. If you don't have the skills to set the new gears up, then you are looking at another $400-$500 to have a shop install them correctly for longevity and then add another $230 to the total cost if they installed the new gears with new bearings and seals, which I'm quite sure the shop would do when installing new gears sets.
Be advised a couple of guys who have relatively recently bought gears from 4Xfourart have made our Zuk communities aware that they have experienced an issue with the gears they bought wearing very prematurely and the metal composition of the gears (too soft) is suspect. Torgot MAY have a production problem that he is dealing with on a "batch" of the 5.72 gears. I know he is aware of the problem. I don't know what the outcome is as of right now.
Also to be fair, both complaints have been on the 5.72 gear sets. The one guy I am aware of (up in Maine) that bought and installed the 4Xfourart 5.85 gears relatively recently, has not, to date, had any complaints of excessive wearing of his new gears.
IF you were to purchase a set from 4Xfourart, it sounds like you would want the 5.85 gears anyway. Torgot (who owns the company) has a long history of off-roading and apparently his products, including his ring gears, are used by many off road enthusiasts in Europe and beyond. He has been around for awhile, well over a decade I believe. You don't stay in business for long if you are a poor business man or a crook. Word travels fast.
So... 4Xfourart is a source for the gears you are describing you want. As far as the price...
IF you could locate a set of the used Suzuki 5.63 gears for sale, I'm pretty sure the owner would be asking around $1,000 for the pair. That's the price I've seen on two sets that went up for sale QUITE awhile ago. Again, that's IF you could find a set for sale. I haven't seen a set for sale on this or any other zuk forum for well over a year now. If it were me, I'd pay the $1,000 for
new gears before spending that much for a set of used gears that probably would have over 150K miles on them. 5.63 gears would probably have 10 bolt s on the rear ring gear too. Your rear diff on your truck probably has 12 bolts. Someone can pipe up if I am wrong on this point.