Craig,
When you (or someone) installed those Urethane bushings on the trailing arms, did you/they lube them inside with Urethane Bushing Grease? If not, they will have dried out in 2000 miles and either be stiff, squeak or both.
And you can’t “just add grease fittings” to the beam, right now, as there is no place for the grease to go. If you intall fittings, they’ll be pushing up against the Urethane bushing with no place for the grease to go. Before you install the fittings (pressed in or threaded), you must drill a hole at the fitting locations through the urethane bushing so the grease can get to the inside of the bushing to lube against the trailing arm shoulder.
Use a drill that fits through the fitting location holes, preferably of a size that you can run a tap into the Urethane that is the same size as the threads on the fitting (or just screw the fitting right in to the Urethane and let it cut its own threads, but you’ll still need a suitably-sized hole, like 3/16” or so). Drill through the Urethane until you hit the trailing arm shoulder and no more. That hole will be the passage to get the grease from your gun to the space between the bushing and arm - shoot it in til it comes out the end of the bushing. It will take a few pumps.
There is special grease for Urethane (“Prothane” sells it) but unless you can get it into a grease gun it might not be for you. I would just get an automotive grease with a high Molybdenum content and have at it. Not quite professional, but it’ll work.
Good luck!