I tried all the suggestions above on my JHH F-4. Toe in the mains, tightening linkage, different expo setups, etc. Nothing helped. It had a bad tendency on rollout to magnify any steering input causing it to go astable and spinout. Not sure if it was due to the dual-wheel on the nose or what. I could taxi but anytime under power (i.e. taking off) the plane was not reliably ground controlled.

A gyro fixed all of this! It tracks straight as an arrow now. Get a normal gyro, DO NOT use a heading lock gyro. Place it as close to the CG as possible. Make sure it is mounted properly, that is if you rotate it to the left the steering corrects to the right. Start off with 1/4 -1/2 on the gain setting. You can use a cheap piezo gyro, but mine was close to the turbine since that is the CG point. So I opted for a more expensive Futaba 240 gyro since they are less temperature sensitive. You could try a normal piezo gyro, I never got around to it. I figure since you are only using it during short periods of time the temperature drift shouldnt be an issue as it would be in a heli. I ended up with around 1/2 gain setting. Too much and it will oscillate, too little gain and the corrections will be too late to help (sluggish). Do some taxi runs and dial in the gain needed until she is stable. Then you shouldnt have to touch the gyro, just let it do its job...

Jim