With the 2nd gen. Escorts, the lack of shifting from 1st gear is a known symptom if your Vehicle Speed Sensor is bad. It could be a bad connector or dirty contacts on the VSS - so remove the connector and try cleaning it. I would use brake cleaner in an aerosol can. And it could also be an internal problem with the VSS, or abraded wires from the connector to the PCM.
I know its not a convenient location. Getting the VSS out of the transaxle is known to be one of the most difficult jobs on an Escort - if the VSS is stuck in the trans case from corrosion. Hopefully they used some grease on that VSS when the plugged it back into place.
I have had VSS's out 5 times. Four of them were when I had the transmissions out of the cars on my workbench; and even so they gave me difficulty. One of them even got destroyed in the removal process. The fifth time was last month when I took it out to check the fluid level in my 5-speed. I had had that one out 2 years & 20,000 miles earlier, and it still soaked up an hour or so on 3 consecutive evenings. Soaking it with penetrating oil wont hurt.
And Escorts with the automatic transmission (an F4EAT) use a cable connected at the throttle body to tell the transmission how much pressure to use when shifting. There isnt a vacuum modulator on them.