'91 YJ . Move the transfer case lever to 4h, the front axle engages and the indicator light comes on. All works fine at slow speeds, but as speed increases (toward 40 mph) the indicator light will flick off and then back on accompanied by "clunk" noise from the front and slight jerk felt in the chassis. Since the shift motor only has 2 vacuum lines, I assume that one applies vacuum for engagement and the other vacuum for disengagement. Both lines lead to the control switch on the transfer case. I suspect the control switch is failing. Any thoughts.