Swapping rear differential gear ratio sizes...

[Henn28]

Just saw your other post so I’ve edited the below…I’ve got a 4.4 but the diffs look the same (except for the rear cover), so I’m assuming the axles are held in with snap rings. On the e53 though their are stub axles that pop into the diff first, then the drive axles bolt to these. Same principle tho.

My first thought is that they will fit, but the snap rings are being difficult. Which they can be. If your axles use the same snap rings that the 4.4 diffs those can be a challenge to “pop” into the carrier, especially when the diff is in the car. If your axel splines are smoothly meshing with the diff splines, but won’t bottom-out with a firm “click”, the. I suspect you just haven’t gotten the snap rings to click into the grooves in the diff.

The case looks the same as the “188k” medium case as the 4.4 or 4.6, but the e53 has stub axles that pop in and out of the diff and which the drive axles bolt to. These are easier to pop into the diff with a rubber mallet. Assuming your axles seat with snap rings, pull one out slightly, turn it 30 degrees and try to reseat it…rinse and repeat. But beyond your issue perhaps stemming from just not getting the snap rings seated, have you pulled the rear cover from that diff and ensured there isn’t any damage inside? It sure looks like the splines are meshing but the snap rings just aren’t popping in, however, I suppose the diff splines could be full of crap from a failure.

As far as other options, only thing that really matters is that the front and rear diffs are the same ratio, or final drive. The front and rear wheels can’t turn at different speeds. Not sure why the part number isn’t on your diff, although it may have been on a sticker when assembled new, but if you just can’t seat the snap rings, you should pull the rear cover on your diff that was working and count both the ring gear and the pinion gear teeth. Dividing the ring gear number by the pinion gear number willl tell you exactly what the final drive ratio is that was working on the car and go find a replacement diff. Since it was working, you can assume that the front diff is the same ratio.

Assuming your diffs had not been swapped previously (did you buy the car used?), I’ve got to think there are plenty of 4.8 rear diffs on eBay you can pick up for fairly cheap. Then you could do the seals on it while it’s easy (not installed) and the. swap it in. If it has been swapped out by a previous owner, maybe to have a different final drive ratio, it could be from a 4.4 or 4.6. The cases look the same, but the rear covers would have had to have been swapped as your pics show a suspension bolt on the rear cover(?). The 4.4/6 has all three suspension points on the case. I do think it highly unlikely that someone went to all the ass-pain of swapping both diffs on a 4.8, just to get a different final drive ratio.

Beyond the above, you could buy eBay front and rear diffs from the same car and swap them in, but the front diff is a big job.