I saw the same info Kuranosuke is thinking of, someplace buried in the old posts over at
http://www.4x4wire.com I believe. I don't know about other models, but I have a factory service manual for my 86 Trooper and it shows a ratio of 6/19 for the speedo gear. The factory service manual for my 79 LUV and it shows 6/20 for both 2wd and 2wd models. Isuzu 4x4 trannys stayed very similar to the LUV up to about 87/88 so I would guess the speedo gear from something before that *should* fit into a LUV.
Now, the Trooper comes with 235/75R15 tires and 4.56 gearing in the axles. The LUV has something like 195/75R14 tires and 4.10 gears. From my figuring the Trooper speedo gear will change what you are reading by around 5%. In my LUV, the speedo is about 12% off with only slightly larger tires, so that would only be about halfway to where you want to be. You'd need a 6/18 to get it right, but I don't know what Isuzu has such a gear. The other option is to take the 4.56s out of a trooper and then you'd be about right on, but that is a lot of work if its just the speedo. This assumes my calculations are right, which they may not be.
Also, just to describe the gear and how to change it. Unscrew the speedo cable from the trans. You will see a round deal where it came off with a bolt just to one side of it. Remove that bolt and the rest of the assembly comes out. It may not be very willing to slide out, but you should be all to turn it back and forth and work it free. Just do not pry up on the area of the part where the bolt was holding it down. (sort of a "tab") That aluminium is not very strong, and it will break very easy. (Trust me on that, done that once myself)
I have swapped gears between an 83 PUP and a LUV tranny and it did work, tho it was long ago and I don't know what ratio the PUP had. I swapped because the gear on the PUP was stripped and the luv gear left it reading slower than actual speed.