More often then not it seems that the speed sensor goes bad. If your cruise control is not working as well it is most likely the sensor. It is a pretty easy replacement; remove the flex pipe between the air box and throttle body, find the one electrical connection on top of the transmission, undo the connector, remove the one M6 (10mm hex head) bolt and twist to remove it. You can find them on e-bay for @ $50 depending on your engine/transmission or go to a junkyard. Nissan want big $$$'s for a new one.
I am pretty sure the manual and automatic sensors are not the same (different gearing). I am not sure if they the same for the DOCH vs. SOCH though. I bought a sensor for my DOCH 5-speed from a guy with an e-bay name of "streetking408ys" (eBay My World - streetking408ys). He seems to specialize in used Maxima parts. May be worth sending him an e-mail? You can also do a used parts search on google. There are a few websites out there that let you put in a description of the part you want and you will get quotes from junkyards around the country.