1. Youve said your centre bearing is shagged out I reaplaced it on my old mans gtp got the drive shaft balenced and it helped quite a bit. Also change the bush inside gearbox extension housing while your there can be the cause 3. Have u checked your tranny fluid level and given it a service? Some times when fluid is low they play up untill the oil is heated I have alway ran my tyre preesures at 40psi. As long as u dont over inflate your tyre and cause uneven tread wear. U may lose some traction but gain in fuel econ.