Had a used rebuilt transmission put in my car recently bouncing speedometer and poor gas mileage

Yes the Vehicle Speed Sensor should be checked. It could have some affect on MPG as engine load demand changes.
The best thing to do is to got to Advance Atuo or Auto Zone and have them pull the trouble codes. They do it for free and a code should come up for the VSS.

If it is getting a bad or non-linear signal from the VSS, the ECU will send the same signal, or it may just be a PCU involved as the cruise control essentially feeds straight off the VSS without much in between, that is erratic to the cruise control module which will try to keep the vehicles speed at the same erratic speed as the signal it gets. It basically mimicks whatever it is set at and trys to keep it at that speed but if can never tell what the speed is or think s it is at that speed because that speed was set at some unstable signal it will jump around with the speedometer. There are only a few feeds that work with the crusie control and the VSS signal is basically calling the shots to the servo that adjusts the throttle and it gets more feedback than it gets input. So, it is not surprising that your cruise control is not working. I think getting the VSS replaced, check the codes first, will get the cruise control working properly.