What type of rear wheel are you using, I have noticed a significant difference between my built wheel CK ISO disc DT spokes Bonty Mustang compared to using a Mavic UST cross roc for example!
Also I have switched to using CK fun bolts which make a huge difference to rear end stiffness ,not that I found my rear ends on either the Moment or ID flex, I weigh 90kgs and ride hard and I am the classic rear end skidder but I have noticed a big difference with fun bolts compared to skewers, this goes for most of the other high end brands as well.
Course won't necessary help with tyre clearance, I personally don't find this a problem at all with the Moment, and when riding Id for I can use 2.3 inch down depending on terrain. Mainly I run a 2.1 rear for general trail riding and a 2.2 front (ID) when racing I use anything from a 2.1,2.0, or 1.9 depending on tyre brand and course type. But tyres are a personal choice
High volume tyres like Tioga are great on the ID 2.1 Blue dragon rear 2.3 yellow Kirin front is a good combo for trail riding.
Later