03 3.0 shake at idel

[rangers92]

my 3.0 has some noticable shake at idel. it also shakes a little more when cold or the a/c is on. i have changed the plugs wires the batery the serp belt the fuel filter i run fuel injector cleaner regularly. an i use 89 gas. the shake calmed down some when i changed the fuel filter. however i am getting great power friom my motor i am getting no missifres no over heating no knock or tick and it revs good. very smoth as well. i have been told all rangers shake specialy the 3.0 something about the resoneces of the unibody and motor?? just looking for insight. also in the summer im rebuliding the motor. all new water pump alt power sterring pump starter and ac compressor. along wiht maybe head gaskets and pistons. motor has 106k on it and starts fine. also while it shakes at idel the rpms stay dead still and constant and only increase a little when the a/c is on. just looking for some insite not worried or anything.