I have a rattle on rear passenger side underneath, I checked the shocks which were replaced 4 years ago and everything else seems tight

Would seem your into the inner doors & have a partial
grasp on things

Now you fix all door windows one shot

#1 Test all switches,ohm meter unplugged or volt meter with power
#2 Test all motors with battery voltage direct -- DC motor will run either direction-- can't hook up wrong
#3 Window Regulators -you know about them (Unless they are broken-you said they are tight)
#4 Do you have good ground side wiring & battery voltage at all
switches & motors
#5 Last resort--unhook the cables & work on freeing up the
glass & window guides etc then you will have found out if the
regulators are the issue

I had an issue with the rear sway bar links that caused a knock and rattle, the sway bar keeps tension on so it is hard to feel for play.

Check for leaking rear shocks or a loose sway bar.

rattle is usually a bad shock, or loose attachment on frame rail . look around pass side frame rail, check sway bar bushings also, see if u can wiggle upper shock cover .

Warranty should be taking care of this problem, but look for missing or damaged rubber parts in the 'anti-roll-bar' mechanism and the rear shock absorber ends. It would help to identify which end of the car it is coming from. Make sure the sliding doors fit tightly. Try 'bouncing' the van at each corner to help identify the noise. The front struts could have an internal problem. Try shaking the front and rear wheels when on the hoist. Hope this helps!

shocks are cheap and easy to replace, its really up to you

It could possibly be a shock or strut bracket.

Check sway bar. On some models as suspension sags, sway bar outer ends will contact the body, leaving a mark or dent just above the end, causing a bang over bumps. Only way to correct this is to change springs or add a spring lift.

you should have rear struts with a leveling system and also strut mounts most likey its your strut mounts