if the needle is jumping its usually a binding speedo cable , the cable drives the pickup bar at the rear of the speedo and even if the speedo were defective the bar still would move based on the speedocable
its not that hard to swap out the speedometer dont even need to pull the cluster for a new one but i would fix the speedo cable first
the lower speedo cable can be removed from the casing under the hood ..separate and remove the little washer in the ctr and then slide the cable out .. if its damaged replace it , otherwise just clean and lube it . the upper needs the speedo removed and then the cable comes out at the dash end
the speedo just pull the dash bezel, remove the 4 screws to the clear plastic , remove screws from fuel panel and pull out the fuel gauge , remove screws and pull the speedo panel forward and then disconnect the cable at rear
