Clunk in the rear-end when applying the brakes in reverse. The clunk becomes less after putting in drive then reverse repeatedly (noticed that when testing with someone outside vehicle to listen for)
Thanks for any ideas!
Based on the tiny little bit you’ve told us about your truck, it could be a LOT of things. Too many for me to type. I suggest you either post a lot more info or take it to a good independent mechanic’s shop.
Could be your rear pads sliding back and forth when going in reverse and then forward. In any case, I would have it checked. Brakes can save your life.
Possibly a bad front U-joint. Checking these for tightness does not really mean much as the joint can be bad and yet still feel tight. Driveshaft removal is the best way to check them and if the shaft is out might as well replace the joints.
IF this turns out to be the case I would suggest using Spicer joints. I’ve had several issues with aftermarket joints from AutoZone, OReillys, and so on.
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The splines where the slip yoke (on driveshaft) enters the trans tail shaft (or transfer case if 4wd) on these trucks are known to bind and need grease applied. This usually shows up as more of a clunk or bump when accelerating from a stop, though, rather than braking in reverse. But I suppose it’s a possibility that’s worth looking into. Search “F150 slip yoke bump” if you’re interested in investigating that as a possibility.
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Good ideas above. Brakes? Could be if they are rear drums, but not so much if they are discs. I think the problem is in the driveshaft somewhere. Ask your shop to remove the driveshaft and inspect/lube it on the bench. Worse case, you’ll still have the clunk, but at least you’ll know it isn’t the driveshaft, and that the driveshaft is now properly lubed. Two piece drive-shafts are difficult to lube properly installed on the vehicle. Double check the rear differential fluid level is ok. If you have a limited slip rear differential, changing the diff fluid makes sense, including adding some fresh friction modifier. Best of luck. Clunks in trucks when stopping and starting at stop signs is a pretty common thing here, you can use the forum search feature to read some of them.