Do not ever go back there. Lazy/Bad diagnosis.
Almost all Nissans have a KS code, and, except for the SC, its something you can ignore for the most part.
Knocking is a symptom of a problem, not the problem per se. Its from the timing being too advanced, whether its from faster burning fuel, the engine's timing, etc.
BTW - Did YOU change the spark plugs and wires, or did a shop?
If YOU didn't do it, there's a good chance the #6 plug was skipped, as its a PITA to reach, and many mechanics simply assume no one will notice one plug way back between the engine and firewall...and it will be gotten by the next guy, etc...
....except that's what the next guy says...so, I see #6 plugs that were installed by Nissan when the rig was made, still there after 200k miles, etc. (So you can be like an Audi, etc, running on 5 cylinders)
Morning sluggishness could be the lubes...some lubes are too viscous when cold, and need time to warm up and flow better/not be so stiff. (Swapping in all synthetic lubes for the engine oil diffs, tranny, etc, tends to cure all that great)
The FLASHING dash light means something else though. As the guy you bought it from lied about things he said he replaced....and, the t-belt would be the most expensive of those to actually do...its the one he's MOST likely to lie about.
If a tooth slips, if the belt breaks, etc...it can do damage...even total the truck by taking out the engine, etc.