The speedometer head sounds like it is stuck or binding if there is little pointer movement. Most greases may have hardened into candlewax during this amount of time. I have also found paper labels fallen into the mechanism causing interference with movement. These mechanisms are delicate, like watch repair, so use care.
The speedometer cable can be removed from the sheath and lubricated while pushing it back in. Vaseline may be better than wheel bearing grease. There was a special speedometer cable lube we used at the dealer:
SD530100.jpg
Does the speedometer cable black box also have wires coming out of it? It may be an early emissions check light switch to turn on a dash EMR light? California emissions truck?
There has to be a pinion drive gear. It may still be in the tailshaft?
There is a chart to select a pinion gear tooth count according to axle ratio and tire size. It is in the service manual. Oversize, non-stock tires may not be included in the chart and would have to be calculated. There may not be a part number for that gear, but get as close to the tooth count as you can.
An example here:
Mopar Speedometer Gear Selection Guide - MyMopar.com
The speedometer cable can be removed from the sheath and lubricated while pushing it back in. Vaseline may be better than wheel bearing grease. There was a special speedometer cable lube we used at the dealer:
SD530100.jpg
Does the speedometer cable black box also have wires coming out of it? It may be an early emissions check light switch to turn on a dash EMR light? California emissions truck?
There has to be a pinion drive gear. It may still be in the tailshaft?
There is a chart to select a pinion gear tooth count according to axle ratio and tire size. It is in the service manual. Oversize, non-stock tires may not be included in the chart and would have to be calculated. There may not be a part number for that gear, but get as close to the tooth count as you can.
An example here:
Mopar Speedometer Gear Selection Guide - MyMopar.com
