I don't see a slip spline on this driveshaft.... Something else need lube?

 

are you hearing this clunk from the rear end do you figure? Have you done any recent work to the truck? rear end components, steering components, brakes? Rotors?

That kind of info might help narrow it down. is it an F150 F250 F350???? 2wd or 4wd?

 

A mechanic friend tells me it's normal backlash after 175k....Maybe so

 

I would not rule out U-joints. back lash can be adjusted for. It can be a bit spendy to do so.

 

Look closer, its the spline. My 2004 f250 fx4 had the exact same issue. Just pull it out and put some grease on it and you will be happy with the result!

 

if it is backlash this would be the first time i have heard about it in a f150 rear axle. im not saying that it is not possible because no truck will have the same maintenance done nor same driver. but i know it is common on the older jeeps with either the dana 35 or chrysler 8.25 on those it is called the chrysler clunk and is normal.
if you have grease and some time there is no harm in pulling the rear drive shaft and putting grease on the out put shaft of the trans where the drive shaft slips on. if it keeps making the same clunk what you can do is check the axle pinion and see if it has any slop in it sometimes the preload on older more high mileage axles get loose and you may be able to tighten the pinion nut just a little and i stress just a little and that may take out some of the clunk if it is still there are you grease the output shaft of the trans where the drive shaft slips on