2001 ford explorer 6 cyl code reader indicates lean mixture on both banks--mechanic stated plenum gasket bad--How hard is this to replace??

Recommend check for a vacuum leak especially at the back of intake where all the vacuum hoses connect. If all the hose seem good then you will need to replace the intake plenum gaskets. Common problem on the Ford 3.8 liter engine.

Code 171 means the fuel mixture is too lean on bank one and code 174 is the same thing for bank two. So I doubt the O2 sensors are faulty. Chances are you have a fuel problem, maybe low fuel pressure.

The error code 1131 means the air/fuel mixture is too lean on Bank-1 while 1151 indicated mixture is lean on Bank-2. You can check for the causes of the problem in following link for both error codes (1131 as well as 1151):

http://www.obd-codes.com/trouble_codes/ford/p1151.php

Well code 170 is for bank one and 174 is for bank two which you probably already know. The computer can not make the mixture rich enough to make the O2 sensors switch to rich.
Have you checked base fuel pressure and watched the data stream coming from the O2 sensors ? Very seldom do two sensors fail at the same time, but it could be something else like the mass air flow meter.

Lean codes on both banks is always a good indicator of a significant vacuum leak. The most common source is leaking intake/plenum gaskets. When its idleing rough spray some carb spray around the plenum where it bolts to the intake. If the idle changes you found the problem.'

Be careful when spraying carb spray. It is flammable!

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

P0171 - Fuel Injection System Too Lean Bank 1
Possible Causes:
- Intake air leaks
- Front Heated oxygen sensor may be faulty
- Injectors may be faulty
- Exhaust gas leaks
- Incorrect fuel pressure
- Lack of fuel
- Mass air flow sensor may be faulty
- Incorrect PCV hose connection

P0174 - Fuel Injection System Too Lean Bank 2
Possible Causes:
- Intake air leaks
- Front Heated oxygen sensor may be faulty
- Injectors may be faulty
- Exhaust gas leaks
- Incorrect fuel pressure
- Lack of fuel
- Mass air flow sensor may be faulty
- Incorrect PCV hose connection

What Causes These Codes: Fuel injection system does not operate properly. The amount of mixture ratio compensation is too large. (The mixture ratio is too rich.) The Powertrain Control Module (PCM) monitors the engine Air/Fuel mixture ratio, the actual mixture ratio can be brought closely to the theoretical mixture ratio based on the mixture ratio feedback signal from the heated oxygen sensors 1. The PCM calculates the necessary compensation to correct the offset between the actual and the theoretical ratios. In case the amount of the compensation value is extremely large (The actual mixture ratio is too lean.), the PCM judges the condition as the fuel injection system malfunction and light up the engine light. The codes is usually set when there is an intake leak after the air flow meter, before replacing any parts check for loose or broken hoses and intake gasket leak. On Ford some models dirty air filter, and dirty or faulty air flow sensor are the cause of the problem.

P0171 and P0174 are lean codes. They do not mean the 02 sensors are bad. The most common cause for this especially when both banks are lean is the plenum gaskets in between the plenum and the heads. They will start to leak and cause a vacuum leak. When they leak bad enough it will flag lean codes and eventually the vehicle will start to idle rough and eventually not idle at all.

Replacing the gaskets will usually fix this problem. Sometime the intake plenum will warp and need to be replaced as well. Replacing the O2 sensors will not fix this problem.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

if the light is flashing it most like hasly some sort of misfire or converter failure, p0171 and p0174 is lean bank 1 and 2, replace you upper and lower plenum gaskets, while they have they off have they replace your ignition coil, plugs and wires, windstars are known for 2 things, lean codes,(plenum gaskets) misfire(bad ignition coil)

have the 02 sensors checked

Look, man here is the deal:
code p0174 - refer to the front cylinder bank
code p0171 - refer to the rear cylinder bank