There was no change in the bolt circle, it's still 6x5.5. The problem is that they went to a FWD style backspacing when they went to IFS on the 4x4 trucks. That draws the wheel in too far for use with the earlier trucks. That change was in late '87, when the '88 GMT400 trucks came out. About the only GMT400 wheel that is both 6 lug and uses the older style backspacing would be the '88-'92 C2500 light duty. The 4x4 used the mentioned FWD offset, and HD 2500s were 8 lug.
