The syntax is awkward. The verse literally reads, And they crushed and oppressed the children of Israel in that year, eighteen years, all the children of Israel who were beyond the Jordan.
It is likely that that year
refers to the initial oppression from both the Philistines and the Ammonites and eighteen years
refers to the duration of the Ammonite oppression east of the Jordan.1 It could otherwise mean that the oppression east of the Jordan lasted for the full eighteen years, culminating in oppression against all Israel (see v. Judges 10:9) in the eighteenth year. Some read the syntax as saying in that eighteenth year,
in which case the oppression occurred for the first seventeen years in the Transjordanian region of the Gilead, but in the eighteenth year it comes to its climax with the Ammonites also crossing the Jordan in force, bringing them into the heart of Western Israel.
2,3
8 and they crushed and oppressed the people of Israel that year. For eighteen years they oppressed all the people of Israel who were beyond the Jordan in the land of the Amorites, which is in Gilead.