How to grease the shackles?

Rod, "the shackles" are not what's welded to the underside of your frame. They are the large movable U-Bolt connection at the rear of the leaf springs that connects the spring to the frame.

They come in many different sizes, but only two styles. The newer shackles have to be removed to re-grease. They generally have nylon bushings and require very little maintenance. They usually have self locking nuts, and you need to make sure they get a drop of RED Loctite on the nuts when you reinstall. Make sure the front springs bushing is in good condition, and the springs do not have any side to side movement.

The older shackles (common up to the 1950's my guess) were pretty trick. They were hollow drilled and had Zerk grease fittings on the end of the bolts for grease maintenance of the bronze bushings. They also usually had castle nuts with cotter pins for adjustment. They NEVER got greased, and the grease dried hard as a rock. I have had to "drill out" the old grease to get them opened, and replace the Zerks.

As with most trailer suspension parts, you will only find them completely worn out, or "just like new".

If you go to a "real spring shop" the cost to replace these parts is very reasonable.  They will have everything needed "in stock".  The go to place for leaf spring parts in the San Gabriel Valley is called Atlas Spring located in the City of Industry.  Ask for Fernando.   In Orange County, the go to place is Deaver Spring (third generation owner) that still operates out of a Blacksmith shop that is one of the oldest buildings in Santa Ana, CA.