Speed sensor(VSS)

Help, I am having problems with my speedometer jumping around. It seems to jump when you get over 50 mph. At first I was told it was a control module in the dash and if when the problem occurs if Ihit down on the dash and make it stop, it was the control module. This mechanic said that there was a bad solder joint on the board and that this unit would have to be replaced. Well I have been driving with this conditon for several months now and the problem seems to be getting worse. Hitting the dash does no good (this was never a 100 % fix when the problem first started). My question is, is it the sensor or the control module? If it is the control moudle what is it called? Reading thru some of the threads on this it sounds matter it sounds like this control module is also my odometer reading. How hard is it to replace this module and how about the cost. If it is part of the odometer can I adjust the milage on it to reflect actual mileage of the van?

Thanks for any input on this.

Daboone