Vehicle shakes during breakaway in reverse with medium to heavy throttle application (i.e. backing very slowly up a steep grade or backing up a trailer). Maybe you need some obd2 scanner for help?
Warm vehicle to normal operating temperature. Locate a relatively steep grade (where it is safe to stop and back up). Try backing the vehicle up very slowly from a full stop. If a sufficiently steep grade is not available, move the vehicle to an open, flat, paved area where it is safe to stop and back up. Apply the parking brake about half way. Shift to reverse and slowly open the throttle until the vehicle just begins to move. Vehicle speed should be no more than a slow walking pace. Extreme care must be taken not to overheat the transmission during this test. Do not open the throttle more than 10 seconds at a time. A low the transmission to cool by running at a fast idle in neutral for at least three minutes between attempts to breakaway during either of these tests, do the following repair:
1986 Passenger Car Application
Fifth Avenue, Grand Fury, and Diplomat
Engine Transmission Torque Converter Number
318 (5.2L) A-999 Low stall/lock up 4431113
This repair involves the replacement of the torque converter.
1. Check engine for any driveability problems. Low carburetor air flow, early EGR valve operation, etc. have been found to be a contributor to reverse shake in some vehicles.
2. Perform a propane adjustment and/or correct any other driveability problems that may exist with the vehicle, using the appropriate Driveability Test Procedure.
3. If correcting any driveability problems with the vehicles does not eliminate the reverse shake, then it will be necessary to replace the torque converter with a new one.
4. Select the appropriate torque,converter for the vehicle from the parts list.
5. Replace the torque converter per the appropriate service manual Instructions.
6. Road test the vehicle and check for leaks.