Transmission does not always shift up and sometimes sarts off in second and speedometer wroks on and off

Transmission not shifting into overdrive right.

Beware speed sensors seldom fail. Chrysler products transmission have a limp mode where they only shift to second gear when something goes wrong and many times when nothing is wrong. If the speedometer fails it goes to limp. If an OBD scan gives a transmission code sometimes clearing the code will end the problem. When driving if the transmission shifts one time as it should the sensor is working.

This needs to be diagnosed on a computer for accurate results.

You probably have a blown fuse that conrols the speedometer and shift point of the transmission; Check the fuses inside the car under the dash on the driver side. The speedometer tells the transmission when to shift.

WELL DIDNT TELL ME THE YEAR OF TRUCK CHECK TRANSMISION FUSE FIRST.IF ALL IS GOOD LOW TRANSMISSION FLUID OR STOPPED UP TRANSMISSION FILTER AND OLD FLUID CAUSE SHIFTING PROBLEMS.FAULTY VSS SENSOR WILL CAUSES TRANSMISSION SHIFT PROBLEMS.IF YOU HAVE NEWER MODEL TRUCK FAULTY TCM OR TRANSMISSION CONTROL MODULE FAULTY.

clearly a gear box fult, its going to be pricy to fix as well

sounds like your vehicle speed sensor is going out. it's the modern equivilent of a speedometer cable. it tells the computer how fast your going which the computer uses to determine shift points, abs function, and speedometer readings. they usually act up a bit before they go out. it confuses the computer. normally located in the extension housing at the rear of the transmission where the spedometer cable would be on an older vehicle

take to transmission shop and have serviced these will sometimes have issues with the solenoids in the transmissions.sooner you get it in the cheaper and less expensive the repair will be.

You have two sensor in the transmisson which is the in put and out put speed sensor, and is located on the left side of the transmission. 
They are the primary input to the transmission control module which control your shift point and speedometer.
They are easy to replace and only held on by one bolt. Disconnect the wire connector and then the bolt. 
Replaceing is in revers and replace any fluid lost. If you could get it to the parts store and have the truck scaned, they could tell if you need both sensor or only one. Before you go out to the parts store, check the wire connections to see if there lose or broken which may be the cause of your trasmission problem.
Good luck and hope this helps and saves you some money

I don't believe the problem is in the vehicle speed sensor. I suspect the problem may be the computer. The speed sensor is just a magnet inside the transmission with an induction coil outside to create an electrical signal every time the transmission output shaft turns. That signal goes to the computer and the computer tells the transmission when and how to shift and displays the speed on your electronic display. I would start by having a reputable transmission shop check out the transmission, then take it to a good mechanic to have him check the computer. It is a relatively expensive fix, but with only 70,000 miles on the Caddy, it should be worth it.