I'm trying to understand what you are asking. 

Ok, you have a BJ40 with a high performance diesel behind it (in US most diesels redline RPM is around 4,000) and your looking for a reduction box? 
But your only running a rear wheel drive at this point?  (2 wheel drive only)

And you want to go slower, faster or increase your torque from your motor?  Because we run lower gears, and/or additional transfer cases to get our wheel speed lower, all the while doubling our torque to the tires.  By reducing our tire speed, we can keep our rpm's up, and tire speed down.  Sometimes too much tire speed will break things.  So, we want to control the speed our tires are trying to grab at things. 
The Toybox is designed to go between the transmission and transfer case.  It isn't designed as the final output to a driveline. 
But this company makes a overdrive/underdrive as a final output.  Which maybe what your looking for.  http://www.gearvendors.com/

We need to know what your plans are for your vehicle to help you figure out what you are needing. 
As for the amount of RPM's the Toybox can handle, that answer needs to come from Marlin or Mike the company owners.