If you floor the gas in a higher gear, like 3rd, while going at a lower speed, like 25, the truck should struggle to accel. If it does that the clutch is holdig good. If you do the same and the RPMs rise but the truck does not accel, then the clutch is slipping and needs replace. The drop problem you are getting is not normaly from the clutch. All cars have a vacuum assist or just a spring on decell, not sure about Xterra, so if you release the gas it does not go down so fast as to kill the engine. These can stick ad the RPMs will come down slow. The only way I can see your problem relates to the clutch is if the through out is not fully disingaging the clutch, which it could be. This could be slave cyl not fully extending. If you get the slow RPM drop when in nutral and you just rev and release gas then it is not the slave cyl. I may be wrong on this, If Xterra has a different system, so others can chime In. Hope this helps.