Unfortunately, the Rally is on indefinite backorder so I picked up a Scorpion Pro F.I.M. instead. Here it is next to the Trakmaster:
As far as I can tell from online specs the Pro is pretty much identical to the Rally. Both are DOT-rated and both appear to use the same compound, although softer compounds are available in the Scorpion line. The main difference is probably that the knobbies on the Pro are shorter than on the Rally, to comply with the F.I.M. standard which restricts knob height to 30mm to try to reduce trail erosion.
Nevertheless, it's still an aggressive tread that should work well in the mud.
I still can't get over how toy-like that Trakmaster looks on the rear. As the 100/100-18, it's one of the smallest and lightest in the line-up. Hopefully that translates to some reliable riding with a loaded bike. Otherwise it's back to the MT43 and hoping that lower pressure and larger diameter make up the traction lost by the smaller knobs.
One other consideration with the smaller rear tire is the weight shift on the suspension towards the rear. When I had the MT43 on, I set my shock for 20mm of static sag and 85mm of rider sag (about 32% of total travel). I'd never gotten around to adjusting this before and immediately found that bike handling improved. Be interested to see how the lower back-end alters handling.
Now, if I can get home some night before dark and when it isn't snowing or freezing rain, I'll be takin'er for a rip there bud!