Rear differential backlash adjustment

[aaaajunk]

On my '83 I have a rear diferential hum while coasting or engine braking . I have tried replacing the oil (80/90 GL5 hypoid gear oil) but the noise persists. My shop manual suggests that there may be a problem with the backlash adjustment. Can anyone fill me in on what exactly is entailed in correcting the backlash adjustment? Is this a major job requiring removal of the rear end, or can it be done in situ through the back plate? I have already had to replace the rear end once with one from a parts car so the present one is from an older car, but if ran fine for 10k or so after the install.

[So Cal Mark]

when you say "older" do you mean the earlier style diff with the drop-out 3rd member? If not and you still have the later style diff, everything is done through the rear cover plate. If the diff was quiet for the last 10k and is now noisy, you probably have a bearing going bad. Backlash is set by shimming the side bearings and by the pinion crush sleeve, so it shouldn't change unless some part is failing, most likely pinion bearings