Welcome to TRS
Good pages to bookmark:
https://www.therangerstation.com/tech_library/index.shtml
And:
https://www.broncocorral.com/tech_library/
You have a 2.9l V6 engine, and an A4LD automatic transmission in your 1990 B2, Rangers used the same setup in the same years
Is it a 2WD or 4x4?
High idle or REVing could be as simple as a Vacuum leak, an old hose has cracked and is leaking
That would cause engine light(CEL) and Lean Code
All gasoline Engine RPMs are controlled by air flow, that's what the throttle plate does, you open it and RPMs go up, close it and they go down, not an old or new thing, been that way since the first gasoline engine in 1890's
So the small right hand pedal in the cab is an AIR Pedal, not really a "gas" pedal

If you just add more fuel you get a flooded engine not higher RPMs, lol
A vacuum leak is by passing the throttle plate, adding more air so RPMs go up while throttle is still closed
You can inspect the hoses and look for problems, maybe a hose even came off
There is a Vacuum manifold on upper intake drivers side, follow the Power Brake booster hose, and check it, to this vacuum manifold
There will be several vacuum hoses connected there, follow each one to its devices or devices(T's on the hose)
Another larger vacuum hose is for the PCV Valve, Valve is located on the top of the drivers side valve cover, has a larger rubber hose attached, check it, this PCV Valve should be replaced every 3 or 4 oil changes
There is a spring on the throttle plate that holds it closed, if it gets loose or breaks then higher RPMs as thottle is pulled open
There is an IAC(idle air control) Valve near the throttle, its an air valve the computer uses to set idle RPMs, its possible for it to fail open, this would cause high RPMs, the 1,500-2,000 area if you have a tachometer
Fuel pump wouldn't cause high RPMs, not sure what they were thinking
Or a dirty fuel filter
Failing or clogged Cat converter would cause loss of power, not higher RPMs
Find another shop
Reminds me of a funny line in several movies and TV shows
"A friend recommended you"
"you just 'think' he's your friend, he's not if he recommended me"