I'm also building up an off road construction semi and found the following Utubes useful;
For spacing out the springs on TLT or F350 front axle;all the parts come from an included kit parts sprue.
https://www.youtube.com/watch?v=XW_b...QteXToZiELTMwW
This one shows road speed with the transmission/transfer box before and after using a gear reducer between the 55T motor and the transmission to slow down the truck more;
You'll need to mount your trans. far enough to the rear to fit the gear reduction in position. You WILL need this GRU if you want to run with your crawler buddies…
https://www.youtube.com/watch?v=osZC...ELTMwW&index=9
For even MORE tandem axle articulation, remove the shocks. But then there might be too much articulation that will let the stock inter-axle dog bone fall out,
and the dog bone will need to be replaced with an aftermarket driveshaft.
You could also go with aftermarket $$$ servo controlled locking axles if that super grease doesn't work out...