With the differential problem in my Jag diagnosed, another one I'm facing before I tackle this problem is when I put the car in Park or Neutral, you can safely rev the engine. If I put it in D or R, pressing the gas pedal down does absolutely nothing. It will only idle. Is this a failsafe? I've got no dash codes, no lights etc.

 

Are you depressing the brake pedal before putting it in gear?