I have a 1994 Cadillac Sedan Deville with a 4.9 V8 in it. I recently replaced the front deferential and deferential housing. I never had transmission problems but now I replaced those two parts the car shifts very hard and rpm’s climb rapidly also the speedometer is not working. After I seen the speedometer was not working I replaced the sensor on the housing and it still does not work and still shifts very hard. What could cause this?

Multiple issues, all coming up after attempted repairs: a electrical connector or harness may have become inadvertently damaged; vacuum harness not installed; etc.

Just go back over each step in the repair and double check all the connectors. Inspect the harnesses to check for broken or crushed wires.