Dinglehoser

Hi all - I just picked up this '88 GL wagon for breadcrumbs and a slap in the rear. It's a single-range 3AT, clean interior and exterior, but it makes a few different noises. There's a howling/whining that seems to be coming from the rear of the car, and changes pitch (but not volume) depending on how fast I'm going. The noise doesn't go away when you go between drive and neutral, accelerating and coasting, or fwd and 4wd. Could it just be that the rear diff needs gear oil, or am I looking at a new rear end? Second question has to do with the horribly short gearing of the auto tranny. How difficult and/or pricy would it be to swap in a 4-speed auto tranny from a GL-10 or something? Thanks.