The tension adjuster on my '97 2.5 was stuck just as bad. I soaked it with PB for hrs and I believe I ended up putting a 1/2" impact on it and when it finally came loose I thought I broke something. And after working on it for 2+ hr's i probably would have been fine with the adjuster breaking so i could just replace it
You have to loosen the idler pulley bolt a few turns if you don't already know before you can turn the adjuster.
Once you do get it free, run the adjuster up and down the bolt with anti-seeze several times so it doesn't stick again. Mine works smoothly now