I have a duel axle home built trailer and at 80 kph it starts to fishtail on curves. At 90 kph it starts on her own. A friend suggested that I try getting a longer receiver (about 2-3 feet) Will that work?

Bob

Check your vehicle and your tow for the correct tire pressure.
What are you towing with? Is it rated for your particular tow?

Check to see what tongue weight is on your tow vehicle. Too much or too little can have serious effects on tow performance.
If the load isn't properly distributed, that, too, will have an effect.
If everything seems normal, there are hydraulic stabilizers available for many vehicles that are designed to counter the effects that you describe.

Good luck, Bob!