my spedometer jumps anywhere from 0-100 MPH. it puts excessive mileage on my truck and i havent been able to figure it out for several months. i bought a new speed sensor and the problem is not solved. looked for any type of shorts or worn through wires, everything looks good. and when i pull down on the 4wd shifter the spedometer jumps all over the place, i pulled the reduction box (between the speed sensor and transfercase in OD models) apart and checked all that out and it all looked good. i cant figure it out, possibly the gauge is messed up? maybe something inside the T-case... if the gauge is messed up what does pulling on the shifter have to do with anything? im stumped...

 

THE 4WD indicator light/sensor on the t-case is on the same circuit as the Speedo for some reason( at least it is on mine).... I would guess you have a fault in there somewhere.