I'd like to use low range in 4x2 to pull heavy load up long slow switchback hill. Is there a way to prevent the transfer case from engaging the front axle, and keep rear axle in low?

check the 4wd solenoids for vacuum hoses fallen off or connectors damaged, underneath by the xfer case~

Since the transfer case locks, you should be able to drop the drive shaft to the front differential, and still have power going to the rear wheel in 4X4 Low. You may find, though, that the transfer case leaks out the seal if the yoke is removed. You may need to find a way to keep the yoke in place in order to prevent that. I would try it on level ground, without load first, though, to see if it will work.

I was hoping to be able to install a switch inside the cab that I can use to disable/enable the front drive train. This would be so that I can use the 4x4 low range gearing in rear wheel drive only. Perhaps a vacuum solenoid or electric switch that could disable the front drive shaft from engaging? Is this possible? Thanks!