my wife drives a 96 98, recently the plenury gasket was replaced and ever since we have got the car back the speedometer goes crazy and the car will over rev and then recently it has gotten worse to the point where the speedo will go nuts and then stay on 0 and the car will rev like its in neutral. The car will then usually stalls or the speedo will shoot up to whatever speed you are at. It doesnt always happen, but i am suspicious it has something to do with the new plenury gasket b/c the car has shifted hard from 1st to 2nd ever since it was changed and it progressively gotten worse to the point where it doesnt seem safe for her to drive with the kids as mentioned above. thank you in advance for any help I need to fix this ASAP and i was hoping someone has perhaps had this problem before.

Do you mean the upper intake plenum gasket? That gasket should have nothing to do with the trans. Although it can cause a vacuum leak, which might explain some of th revving.

A non working speedo, could be a VSS sensor connection, which might have gotten tugged on when the wiring harness was moved around.

I believe that car has a 4t60 transmission, and that has a vacuum line going in to the front drivers side. Check that. That wont explain the speedo problem. Like Bill mentioned, check the VSS plug. That'* on the passenger side on the diff cover.