Is the S918A rear end a suitable replacement for S829A for my '99 F-150 4x4?
same gear ratio 3.55. but ring gear in the S918A is a 9.75

 

OK... let me ask this a different way. The S829A is a 8.8" ring with a 3.55 gear ratio.
The S918A is a 9.75" ring with a 3.55 gear ratio. It is not considered a "direct" replacement, but the S918A has come out of a '99 F-150 like mine. I'm afraid the penalty for a mistake here is a blown up transfer case, which I would like to avoid.

 

Probably would work as long as you don't beat the living he!! out of your truck.

 

i had that problem. i have a 97 f-150 and the vent hoses were pluged badly; and the transfer had a recall the vent hoses were leting large amounts of water enter through the vent. i just ran a vacum hose higher up and made it loop back down with a filter on the end. when the transfer gets hot and cools down it breaths sucking air in when cold and pushing it out when hot. the vent on the rear axle will be pluged probly at the bottom where it goes in to the axle.