Didn't know if anyone knew the backspacing of a factory wheel on a '91 4runner-- May try to upgrade rims one day soon, and would really only like to be an inch or so wider than factory. I'm hoping this will give me a little wider stance, but not too wide. Thanks

 

I think it's 5", but a search would show for sure.

 

I thought about 4 1/2 inches

Measure yours like this

see post 13 in this

https://www.yotatech.com/forums/f180/15-x-7-wheels-79939/

The solid axle trucks are about 3 1/2 inch BS (so on yours they would stick out about 1 inch farther on each side)

 

The stock wheels are 4.5".

The most common replacement is 3.75".