Hey everyone. I've recently completed a 1.9l Volkswagen tdi swap in my 1991 Toyota hiace and am doing some road testing with it. It's a 4wd van with a g52 transmission and g144 axle. From what I understand the front and rear axle has the 8" 4.88 ring gear(although if someone could clarify that for me that'd be awesome)
I encountered a problem today with speeds above 100km/h: the engine is revving around 3400rpm in 5th gear and was hoping that swapping out the ring gear to a 4.10 would help to lower the rpms at highway speed. My question is would I be able to swap out the ring and pinion and 3rd member(essentially the whole diff) for a 4.10 gear? And would the smaller 7.5" ring gears fit into the diff housing? What vehicle could I get the front and rear diffs from to go into the diff housing?
I don't have a ton of experience with differentials so I'm not entirely sure what to look for, any help is much appreciated!

Here's the van for reference!

 

I have no experience with yota vans. But.... if it’s the same 8” dif it’s cheaper to buy a used one and swap into van than rebuilding. 7.5” will not work in 8” housing.