I’d be curious what the downsides are compared to 05+ super duty 60.
-Metric bolt pattern, whatever. They can be redrilled
-unit bearings....they have proven to be stout on 05+ Are 99-04 similar?
Doing 30 spline to 35 spline outers is a bigger PITA then the old school traditional spindle. The old school stuff is a bolt in ordeal. 99-04 requires machining....are the machining costs offsetting just getting a 05+? I figure it’s free/cheap if you can do it local but having to buy new unit bearings already machined isn’t cheap.
They are narrower then 05+ if I remember correctly so that can be a plus.
'99-'04 is 69" WMS width vs. 72" WMS for '05+
'99-'04 has weaker unit bearings than '05+ that need to be bored out to fit 35 spline outers
'99-'04 has stock 30 spline outers vs. factory 35 spline for '05+
'99-'04 has a smaller ball joint spread (same as '92-'97) than '05+ so the ball joints wear more quickly with big tires and heavy rigs and they will not accept 1550 U-joints like the '05+ stuff (The later ~'13+ axles came with 1550 joints from the factory).
'99-'04 has smaller brakes than the '05+ axles, but that can be a good thing if you want to run 16" wheels for some reason.
'99-'04 housings are much easier to work with since you don't have to cut a huge piece of the centersection casting off if you aren't using the factory radius arm mounts on the '05+ stuff and they have more driver side tube.
A '99-'04 axle is a good axle, it is just that the '05+ stuff is plentiful and is better in almost every way. It will still be a massive upgrade over any Dana 44 axle. My biggest hesitation to run the '99-'04 axle is just the machine work involved on a wear part to run 35 spline outers which you probably want since 30 spline outers are usually the first thing that breaks on a factory axle. Unless you are running huge tires on a fairly heavy rig, the unit bearings aren't really that big of an issue, just don't use cheap part store replacement bearings. For what it's worth, the original bearings on my 7.3L that has been on 33" tires since before I owned it lasted almost 200,000 miles before I had to replace them.