engine vibration under load- injectors?

[vanordinaire]

My 04 Ford Ranger has started to vibrate at low revs under load (mostly when pulling away up a hill. It doesn't seem to be anything drivetrain related and feels like it's the engine. It smooths out when the revs rise and the load drops.
The only other thing I've noticed is a bit more smoke when starting from cold.
No sign of leaks.
I'm thinking possible injector fault but not sure how to confirm or which injector as there is no sign of a fault when sitting at tickover.
Any suggestions?

[vanordinaire]

Thanks all who replied with helpful advice. I followed the general consensus, poured a bottle of injector cleaner into the tank and 300 miles later, it's running like a sweetie.