as was mentioned... more information on the truck model and serial would be alot more helpful but i might can suggest some things based on the system it has.
From your description it being a nissan based system here are a couple things to check on based on my experience from working on these systems

1- check the alternator charging output, make sure the alternator output is not overcharging. If there is an overcurrent condition going to the fuel system components it could cause false readings in the ecm creating a condition causing the ecm to over compensate for the irregular voltages making the engine surge.

2- if by some strange chance the ecm has been swapped out the settings for the gear ratio's may need to be adjusted, not all trucks are the same and incorrect settings can cause the engine to hunt while travelling.

3- the ETC (electronic throttle control) may be failing, failed spark plugs or fuel lock stuck open. Try cleaning the fuel lock and make sure you have OEM spark plugs installed and they are in good condition. Using the diagnostic program monitor the ETC output for irregular operation , if ETC is not fully opening and closing (0 to 100%) during operation it should be cleaned and if gear housing is accessible relubed, or unit should replaced.

4- check and clean injector(s) as was suggested previously (edwardt) , and check fuel pressures also.

these are just a few things that i know of that can cause engine surging on these nissan systems.

good luck

As there many variations of engine and transmission for this model as previously said more information would help

complete model and serial number helps get better opinions...
check the injectors, especially the o-ring around the smaller injector.