The stud pattern on all toyota 4x4's from 79 to 07 are 6 lug on 5.5" bolt pattern, the same as 1/2 ton chevy, nissan and Isuzu 4x4 patterns. (new Landcruisers have a 5 lug) To measure your backspacing lay a straightedge across the back of the rim. Measure with a ruler the distance from the straightedge to the wheel mounting surface. this distance is your backspacing.
For instance, the backspacing on most 15x10" or 15x8" rims should have a backspacing of 3 5/8" for a toyota application. I don't know what a factory 15x6 or 15x7 would have but its probably very close to this.