first question, is your track loose???
if not, your track can bind causing the suspension to feel stiff.
second, what is your total gaps? 14-16 is somewhere to start, 60-40 gap is what I like to end up at, 50/50 would be with a full tank of gas, all gear and a meal. and yes, once you jump on it and the rod engages, it will get real stiff because your now instantly using both shocks with no transition between. I would guess pending more info that that rear shock preload needs to increase.
either lower spacer will work to some degree, I don't think that is the issue.
the long track extension should not in any way change your settings on the floor.
out in the trail, you may find you dont want that much transfer, stock viper with a 144 is alot of hookup,
When you sit on the one with the pioneer setup, what happens with the front of the sled? my guess is, they set preload higher than I like to start with on the center shock. This will cause the front end to lift when you sit on it. Not bad, just different ways to go. I am a flatlander and will always er on the side of lower. what did they recomend for transfer gap?
Need more information.