rear end conversion or ratio change?

[Tim240Z]

Great to have the search feature back!!
Anyways.....I have a 99 5.7L and 4L60E on a pallette in the garage waiting to go into the truck, but it appears that I have a 'corporate'/eaton/spicer diff out back in the truck. From the searches I've done, it also appears that the ratio is 4.1 or higher (numerically), and I am doing the conversion to get the best fuel consumption possible (removing my 360hp 350/TH350).

So, I would like to get some better ratio gears to that end (maybe 3.7, 3.05 etc). It also seems that gearsets for these rears are impossible to find (?), so my questions are:
Are gearsets available?
or
Is there another rear that I can use (maybe from a newer truck with 8 lug)?
My truck is 3/4 ton with coils....

Thanks,
Tim