Well, you can't service anything in the rear diff until the axle shafts are out either. But if the front had an access/inspection cover, wouldn't you have the same options for diff-still-in-axle work in the front as you do in the rear?
Toyota Land Cruisers, Corporate GM, Dana or other manufactures use a removable cover on the rear axle housing to permit access to the differential, in order to remove the axle shaft retaining components, read semi-float c-clip axle designs found in Land Cruiser 40/55/60/62 series found in the USA. GM and Dana have both used c-clip axle shaft retention methods in the past, even as recent as the TJ Wrangler with the Dana 35, and the Corporate 10 and 12 bolts found in Tahoes, pickups, and other GM lighter truck products.
There are other manufactures that use a semi-float axle design that do not use c-clips to retain the axles, but rather, a bolt on retainer that keeps the bearing assembly that is pressed onto the axle shaft, attached to the end of the axle housing using bolts and nuts. Ford 9” and the 8.8” Ford, along with the Toyota 8” mini truck rear axles are a couple examples.
Dana makes semi-float rear axles that use the same style of bearing retainer keeping the bearing and axle assembly secured to the end of the axle flange with bolts. However, they do not use the third-member/center section design, which necessitates the removable differential cover in order to service the carrier assembly for the ring gear.
You do not need to have a removable differential cover on an axle that uses the third-member or center section design differential. If there were a justifiable reason for having a removable cover on a full-float axle housing, Toyota, Ford and others would have made it so. (Land Cruiser full-floats, 80/100 series in the USA) There is not anything that is really serviceable on those differentials. In the event your spider gears fail, they will likely find their way into the ring and pinion, only to ruin more components, resulting in the need to remove the center section from the axle housing for cleaning and repair. There is not enough room in a Land Cruiser axle housing to set up a differential and properly adjust carrier-bearing backlash with the differential center section mounted in the axle housing.
The Danas: Any idea why Dana's design put the seals so far inboard
When Dana changed to an open knuckle and exposed u-joint instead of a closed knuckle/u-joint or CV affair, they figured there was no reason for oil to be out by the knuckle if there is nothing to lubricate there.
Do the shafts rust between those inboard seals and the seal at the end of the tube?
The axle shafts rust and the tubes collect all sorts of dirt/mud/sand/abrasives, etc. There are not any seals on the outboard section of the axle housing preventing debris from getting into the axle tube.
Does that system make seal failures more difficult to notice?
No, not really. When the inner axle seal is leaking, it just weeps down the axle tube and makes a mess at the knuckle, very similar to what happens when a inner axle seal is leaking on a Land Cruiser or mini truck front axle assembly.
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