There is only one on the rear diff of these trucks, which reads off a tone ring bolted to the back of the carrier. The DRM stands for Dorman, and that's the same part number listed above for $12 from Orielly's. In '98 that sensor, through the ABS system, gives the PCM the signal for the speedometer. On earlier trucks, the Speedo is fed through a field generator on the tailhousing of the transmission of T-case.