Offset depends upon what width you are looking at, it changes based on width. Specifically, a 10" wheel with a -25 offset will have the SAME 4.5 inch backspace as a 12" wheel with a -50 offset. Backspace for 99-04 is in the 4.25 to 4.5 range.
This should help: Custom rims, wheel tire packages for your ride - RIMSnTIRES.com
We have 38x15.50-16s on our 1/2 ton mud truck, I'd really like to put them on 12" wheels because on 10 inch wheels we have to run 25 psi to get a decent tread contact patch. Any more air than that and they start standing up in the middle.