Get the whining checked out
Get the whining checked out, I purchased my d-90 about 3 years ago (95 as well), & it had a similar whine / noise, come to find out the T-case needed to be replaced. The whine was defiantly present around 2 or 3 gear during 40 to 50 m-p-h. When you take it in have them check the speed"o" - meters transducer as well, after getting the T-case replace this transducer made the truck stall (bad tooth on gear) & gave a similar whine.
Just trying to help out?