I haven't dealt with the ones on my T4R yet but my wife's van required an "intervention" a while back. Some sound deadening in the door had popped loose and had jammed up the regulator.
If the window is still going up/down, but just slowly I'd bet on something binding/dragging. I think there are some plastic guides that the glass rides on. Maybe one of those popped loose and the window is dragging (more friction). Does the motor sound particularly more "strained" (is it louder maybe too) than the other side?
Getting the bits and pieces in/out is a royal pain in the butt and I believe the regulator is specific to each door but the motor is interchangeable. That's a lot of effort to "troubleshoot". I'd concentrate on the broken door. Maybe pull the panel and inspect everything you can see. Look around the bottom inside of the door for any screws that have rattled out, broken plastic pieces etc. You might need the passenger side door as a "reference" if you find something that looks odd. Then you can look at the working one to see what it's supposed to look like.
Not sure on the T4R but on the van I thought the glass and regulator were one unit and the drive system disconnected easy. Instead, it was exactly the opposite. The whole regulator (scissor lift looking piece) and the drive motor are integrated. The glass disconnected with a handful of bolts. Thankfully toyota made the drain holes in the door big enough to fish a bolt through since I dropped one back into the cavity during reassembly.
Be careful with this setup. Door glass is heavy and that regulator is like a big guillotine....it will pinch the crap out of you (guess how I know). I used a soft-faced clamp around the door frame onto the glass to hold it in place while working.
Good luck....post pics if you find the "smoking gun"....
