I currently have a 98 Ram 2 wheel drive with 391 posi, this truck has reached the end of its life and I would like to swap the 391 rear axle housing into a 99 Ram with 355 rear gear. Will the speed sensor on the axle housing tell the computer what it needs to know for the speedometer to read correct or is there something else I need to do? I'd appreciate any input on this, thanks.

There is another speed sensor at the transmission so I think the speedo will be in error. Because both ratios were available in that year, the dealer can flash the PCM to accept the different ratio.

I have swapped them like that in a 98 and it was fine with no flash. The sensor on the trans is for the trans to shift based on output shaft RPM just like the old centrifugal governors did, and the one on the rear is for the speedo and ABS, the one on the rear goes the same RPM as the rear tires.