Backing the GTS Into a parking space today and I was getting popping sound coming from the rear end. Third member is a 741 case w 3.91 and a Suregrip. It came with the the car and howls a bit. We checked the gear wear pattern and it looked good.

I was thinking it's may be several things

1. Needs more whale oil added. Clutches are making the popping sound. I need to check fluid level

2. Axle end play is off. In my mind this should only affect the wheel bearing

I should note it doesn't make a sound going forward right or left. Only backing straight, right and left.

Opinions?

The PO may not have used whale oil. unless the Doc or someone advises otherwise that'd be what I'd do (cheap/easy) right off the bat

I use the term whale oil but I know it's posi additive. That's the old school speaking.
Yeah, talked with several others and they say the same think. Only does it while backing.

Go to your nearest Ford dealer that has their posi additive in stock and buy it, make sure there is enough room in your rear end for the entire container and put it in and drive it to see if that fixes it or not

If the clutches were stacked in the wrong order, it will be too tight. I had one that would pop and bang when warm after driving but it was quiet when cold out of the garage. The additive is a good thing to try, but if it keeps up it is hard on the spider gears to pop.

This was the original pig that was put in the car. This was verified with the original owner. The car drove around all through the 70s, 80's and 90's. My guess based on conversations last night it needs some additive. I put a tube of Stalube additive in when I filled the pumpkin but that may not have been enough.

I do appriciate all the advice so far.

We have used the sta lube additive many times without issue
A long shot, make sure the pinion nut is tight,
Otherwise a worn differential (cone or clutches) would be my guess.

Can someone please tell me how do the clutches determine if they are going forward or reverse ... in a straight line ?

How do know it's the clutches? My bet is it the cross shaft worn in the case. That's why it changes backing up, everything is loaded the opposite direction. How did you check the pattern? It can't be dead on if it howls. I say pull it, disassemble and inspect.
Doug

It can howl if the carrier bearing is worn.

The carrier was was pulled out and cleaned and powder coated. I throughly clean the inside before reassembly. When I re-filled the housing I only used one tube. That may have not been enough.

Howl is usually the pattern, incorrect pinion depth/backlash. Bad bearings will growl. A loose set-up, (no preload) can cause howl due to the pinion or carrier moving. I can see the clutches possibly snapping but on a non worn correctly set-up sure grip this is seldom a problem. I agree clutches don't know forward from backward unless something is worn out. The two prime candidates are the cross shaft hole worn in the case. If it is a cone type the cones are worn causing contact at the outer edge of the cone.
Doug

^^^ This. Time to rebuild before the spider gears give out (DAMHIK).