Hi Sir or Madam, I am John Mc, a Ford, Lincoln, Mercury dealer technician.I hope I can help solve your problem quickly and accurately.
Try shifting to D or turning off the od switch (these cars were equipped with both systems) If the rpms do not change significantly then you are not going into od. If this is the case you probably have a blown out valve in your valve body. This was a common issue on these year models. If this is not the case then you may just have a broken od band in the transmission and fixing this involves removing the transmission and disassembling the unit for repair.
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At 55 mph 1850-1970 rpm.
Sorry to say but to even remove the valve body you have to do a lot of work. It is under the side pan on the drivers side of the transmission. Nothing cheap to repair these transmissions. First thing to do would be to inspect the valve body for a valve blown out on the side. If all these are ok you will need to remove and disassemble the transmission. If this is too expensive a repair it may be best to replace the transmission with a salvage yard unit.