I just replaced one on a 77 Minuet. If the 66 has a Henchen axle it is a breeze.
Find the jacking points, put it up on jack stands, remove the wheels, remove the brake wires. unbolt the shocks, support the axle in the middle with a floor jack, 4 bolts and lower the axle, remove the shocks from the posts.
To replace just reverse the order.
Took me about 1 1/2 hours with the help of a 1/2" impact wrench.
If the trailer has springs it should be just about the same procedure.
The big thing to practice is SAFETY. Do not get under the trailer for any reason when it is up on stands.
Get yourself another body, then you will need some flat level solid ground.
Procure some good quality jack stands. Pay attention to where you are placeing the stands.
Every thing else is easy.