I have a 1998 mercury mountaineer and it shudders when shifting so bad it feels like I'm running over those cuts in the highway designed to wake people up.I feel it is the lock up torque converter and I would like to disconnect it to see if the problem goes away. Does anyone know an easy way to shut it off or disconnect it? If I accelerate hard it shifts beautifully or if I accelerate soft it shift beautiful. When I drive normal it shudders.

SOURCE: 99 Mercury sable speedo/tranny

VSS (vehicle speed sensor) Tells the PCM (computer) the speed of the vehicle so the PCM can make the proper information to the TCM (transmission control module) to shift.
YOU are goin gto need a scan tool NOt a code reader to monitor the VSS to see what it is putting out for speed when the car is moving But I believe you need a VSS but do not not Go out and buy one until you have had it tested , and can positively confrim it is bad first. Please rate this a fixya if you find this advice helpful, Thanks Randy

SOURCE: My 2003 Mercury Mountaineer Gear Shift is stuck.

See if u have brake lights. This is what energizes the shifter solenoid. There are 2 fuses u can check both are in the under dash fuse panel. Fuse 23 (15amp) and fuse 26 (7.5amp).
To get it out of park turn igntion switch on stopping just short of engerizing the electronics then u should be able to get it out of park and into neutral.

SOURCE: P1744 TCC on a 2000 Mercury Cougar

On that model it is almost always the torque convertor.

SOURCE: My 2003 Mercury Mountaineer Gear Shift is stuck.

THERE IS A SENSOR ON THE BRAKE PEDAL THAT RELEASES THE THE SHIFTER.THERE A 3 COMPONENTS INVOLVED.U CAN PUT IT IN NEUTRAL BEFORE U START THE CAR.THEN WIL SHIFT.

SOURCE: 2000 Mercury Grand Marquis has intermittent

TOOK MY 2001 GRAND MARQUIS TO THE TRANS SHOP AND OWNER, 35 YEARS IN THE BUSINESS, SAID IT WAS NOT THE TRANSMISSION. IT IS AN INTERMITTENT ELECTRICAL PROBLEM. MOST LIKELY A COIL ON PLUG THAT IS FAILING BUT NOT ENOUGH TO CAUSE THE CHECK ENGINE LIGHT TO COME ON YET. DON'T PUT ANY OF THE "FIX-IT" TRASH PRODUCT IN YOUR TRANSMISSION, CAN GUM IT UP OR DISLODGE DIRT OR METAL FRAGMENTS THAT CAN CAUSE MORE DAMAGE. SOME GARAGES HAVE COMPUTERS THAT CAN DO A STRESS TEST ON THE MOTOR TO FIND WHERE THE WEAKNESS IS LOCATED AND WHAT PART IS BEGINNING TO FAIL. IF IT IS A COIL THE PART, IF YOU ARE HANDY, YOU CAN CHANGE YOURSELF BUT EVEN IF YOU ARE NOT IT IS NOT AN EXPENSIVE DIAGNOSIS AND FIX.