Sounds to me like something is going haywire internally... worn part is partially engaging/disengaging at will...

BTW... niether my '92 XJ Laredo (4.0/AX15/NP231) nor my current '94 XJ Sport (4.0/AW4/NV242) had vacuum disco D30... my '93 Wrangler Renegade (4.0HO/AX15?NP231) didn't either... switch must have been befoer '92... FWIW