Yukon 4WD indicator light Flickers

:screwy: 1991 Yukon 4WD indicator light built into floor shifter flickers in Hi Range. Solid - ON when in low range. Can't be sure if 4wd drive is fully engaged when in high range. It almost seems as if it is only partially engaged and slips in and out.

Any thoughts ?? Is there an adjustment ? Is the 4wd engaged via a vacuum actator that might be going south ?(where is it located?)

Thanks :lol2: