no reverse 96 surburban 4x4

My wifes surburban has lost reverse and whines in drive,,, I have read about others with a "clunk" in driveline. This truck just started
that about a week ago. Would this have been a prelude to deeper troubles? Trans fluid is up and clean (no burnt smell) I have noticed that it has always had "gear noise" in the rear for years. louder at higher speeds and deeper at low speeds, I figured that this is pinion bearings etc. Tranny will make a whirring sound an will stop when engages in drive,, but just whirr in reverse. which way do i need to turn next??
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