98 v6 to a 95 v6?

[84LX89GT]

i believe that the 3.8's went to a split port design in '99 which is a harder swap because the heads are different, but the '98 should fit fine. you'll just have to use your intake manifold and all the sensors off your old engine since your '95 is eec-iv and '96+ engines are eec-v.
as for the rear axle, if you change the ratio, you'll need to change the speedometer gear, if it's a hall effect sensor instead of a gear you can't do much about it to get the right speedometer reading. the rear axles are both 7.5" ring gear axles with the same size rear brakes.