i have a 95 ram with the 5.9 i recently replaced the transmission in the truck...about a month after replacing my speedometer started acting up. it will jump around and then evetually drop down to 0. this will cause my transmission not to shift so i my truck runs around 2k at about 50mph...and then evetually my check engine light will pop on. i would have it looked at but i have reason to believe a previous owner removed the plug in for a code reader. this problem does not happen all the time. it will go a week where its just fine and then 2 or 3 days of the issue...any ideas?

 

VSS is in the back of the trans, or, in the output shaft housing of the T-case. Check wiring.... if wiring is ok, replace the sensor.

 

The connector or wiring is probably crudded up.