I have had to shove a 21 inch tube in the rear tire before, and I've changed plenty of flats on the trail. The 21" tubes in the rear tire work OK in the dirt but their a little weird feeling, especially cornering, once you get back out on the road. But it's way better than running flat. Definiately get the HD tubes. I put a rear rim lock on my 09 and yes it does throw the tire out of balance. Since I mainly ride off road with this bike I haven't put on any balancing weights but I probably should.

 

I threw rim locks on both wheels. I had to have them balanced as it was just too awful riding without some sort of counterbalance. In hindsight I wish I hadn't put one on the front but I'm glad I did the back.

 

Apart from all the advice on this thread the only other thing I can add is leave the valve nut loose under the cap. I ripped 2 tubes (inc a hd one) before learning this trick. Haven't had a problem since. 1 rim lock on the back and balance it (if riding over 50mph often), I don't need one on the front but it depends on what you are doing with the bike.