My 1990 LX 5.0, T-5Z has a thrumming sound vibration, only in 5th gear starting at 55 m.p.h. and growing louder as I go faster. Seems to come from the rear. Trans. mount and aluminum drive shaft are good so, as it is origional, I suspect the rear pinion flange. Also a slight drip is visable there. The part isn't expensive and I have a friend with a lift. What do you think?
It's hard to say for sure if this will fix your problem, but it would be worth a shot. I had an '89 LX and after 3.73 gears were installed I was getting a howling noise around 60mph. I thought it was installer error at first, but I had a leak as well. I replaced the pinion flange and never had another sound or leak come from the rear end after that.