1989 Dodge B350 Fuel injected 5.9L Starts right up, runs alright. Then when you get up over 45mph, it starts to knock. You let off the gas & it stops knocking. Give it the gas, and it starts again.

Multiple choices - depending on how the engine power changes during the knocking...
If power remains, then your timing is too advanced. That could be caused by a bad chip or a bad initial distributer setting. (Check the engine timing).

If power lessens, they it is likely that you are running lean - due to a lack of fuel pressure. Causes: Fuel filter clogged, fuel pump failing, fuel pressure regulator failing.

Can think of no other causes. Check these and I suspect you find the fault and the cure.
-Jim