A 6 lug 4x4 Blazer should have 7/16" x 20 thread lugs on all 4 wheels..
A 2wd would have 5 lugs with 1/2" x 20 threads on all 4 wheels..
If the rears are different there is a possibility the rear axle was swapped for a later version that used metric lug nuts & studs,or someone got the wrong ones when the originals were replaced,or some other modification was done..