Once again this is a case where it would be nice to have the specs on your truck, such as, is it and auto or manual?
Here is the information from US Gear for a manual Transmission:
MANUALS - As with automatics, the internal shift design of the unit (and the mechanics of the shifting) remain the same; however, the method of electronically inducing the shift mechanism does, in fact change.
To shift the Dual Range:
1) While the vehicle is moving and the driveline is loaded, either pull UP or push DOWN on the shift actuation switch connected to the transmission lever.
2) WAIT APPROXIMATELY 2 SECONDS! This will allow the Dual Range shift assembly to load.
3) Quickly, flash the clutch pedal to briefly relax the driveline pressure. Complete depression of the clutch pedal is not required and can cause substantial momentum loss.
Again a small CLUNK and a change in engine RPM should be noticeable upon the gear change.
*** NOTE ***
If using the Dual Range to split-shift in the manual applications, at least 2-seconds should still be given for the spring assembly to load prior to clutch depression. However, because the clutch depression during the shift of the manual transmission will completely relax the driveline, it is not necessary that the primary transmission be shifted to the higher gear before the Dual Range is engaged.
It is very important in any application that the shift assembly of the unit be allowed to load before the driveline pressure is relaxed. Failure to do so will cause all of the shift load to be transferred to the shift motor, which is not designed to accept this load on a prolonged basis.
By providing for the use of additional gearing with minimal effort, the Dual Range Auxiliary Transmission offers complementary driving ratios to, in most cases, an already over-taxed driveline. By using gearing as a "partner" with your engine's horsepower, properly operated, the Dual Range can give you miles of worry-free service and substantial gains in both performance and economy.