Verify the 4x4 works. Not sure about the 91, but my friends 88 was vacuum actuated and the line swelled from oil on them and wouldn't actuate the front end.
My 90 had the vacuum disconnect front axle as well and yes, oil from changing the engine oil filter would drain on the hoses and swell them up. Also the 90 degree oil filter adapter is prone to leaking over time from changing the filter, quick fix though.