98 F150 4x4 4.6 Ext Cab/SBox Rear End went. Appears to be inner left side bearing failure.

How complicated to rebuild?

Initial used quote for an H9 (3.55, 8.8) was $850 complete, $500 housing/gear only (no shafts).

 

They are complicated enough that you should get a professional to rebuild it. If the carrier bearing has failed, the ring gear/pinion gear mating i probably worn in such a way they will need to be replaced. I'd go for the $500 used housing and reuse your axles or get a pro to rebuild yours. I'd suggest checking out a 4 wheel drive shop because they change gears a lot and should know what they are doing.