It is infinitely easier to change the rear torsion bar grommets now with the body off.
In fact, it is nearly impossible to swap them with the body on unless you cut one side of the grommet to get it around the torsion bar and then glue them back together with super glue.
Go for the stock rubber grommets on the rear. The Urethane jobbies will give you a harsher ride and probably crack themselves into oblivion in a few years.
Don't forget to lubricate them with either talcum powder or CV joint grease when you install them.