Sounds like this is gonna be a sweet project! Best of luck with it MetalMan, I'll be watching for sure. IFS with a solid rear axle is always very cool on scale trucks IMO. I'd cetainly go for a single purpose-built axle in the rear, for durability and realism over a more complex and possibly-fragile conversion of the Slash gearbox/suspension.
Incidentally, have you ever seen Thorbob's 4WD CORR-ish truck that he built a few years ago? It's an awesome truck that also used IFS and a solid rear, so you might find it useful:
http://www.thorbob.com/baja_project_intro.htm.
