Your speedo works off a gear and not a sensor. The cable goes to the transfer case and is held in with one 3/8" bolt. Remove this bolt and retainer and pull the cable out to inspect the gear. If the gear is bad then you must replace it. Chose the one with the same color as these are specific to vehicles. Different colors are available to compensate for different gear ratios or tire diameters.
If the gear looks fine then you must inspect the cable itself. If the cable has broken within the housing then you will have to replace the entire cable. Pulling on the speedo cable itself (the inner portion) can help determine whether or not its busted.
You must also make sure that the cable is plugged into the dash cluster itself. Be careful if you handle these old clusters. The white part is fragile and may break easily. Do some verificatoions of this before we discuss the replacement of the cluster. If you have to replace it, you will be looking for one out of an 80's van. More on that later should we find the need.
OK, the other gauges. Your gas gauge recieves small voltage through the sending unit, and you have to make sure that the wires are clean and not shorted anywhere.
You issue can be a couple of things, either a bad sending unit, or even a bad circuit board in the dash cluster. Since all of the gauges appear to have a problem then I may lean towards the fact that the printed curcuits in the cluster are going bad.