Hello, I have the same problem with my 88 jeep. I am going to replace the unit with the 4x4 Posi Lock from Tellico4x4.com.It is easy to install and costs about $200.00.
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ey bud...i live in fredericton...pretty close to you and i am swapping out my axles so if ya want i could sell you my vacum shifter...give me an e-mail....west747@nb.sympatico.ca[addsig]