ESR is indeed a non-specific marker of inflammation. Numerous things can cause a high ESR.
The problem with steroids is that while they are fantastic at reducing inflammation, they come with a plethora of side effects. GMIL has developed obesity and diabetes secondary to steroid use, and her adrenal glands are beginning to pack up as well. Of course the steroids aren't doing her blood pressure any favours, and due to the high blood sugars caused by the steroids, her diabetes can't be properly controlled. Sadly she's effectively a walking mess, mainly as a result of the steroids. The difficulty is that her PMR has returned with a vengeance every time they've tried to wean her off steroids.
PSA: if you're on steroids, DO NOT stop them cold turkey if you develop side effects. Steroids mimic the effects of cortisol, an adrenal gland hormone. Consequently, your body senses that it has enough cortisol and stops making it. It takes a few days to recover the ability to make it. This is why, unless you're on steroids for a very short time, you're usually weaned off slowly. Stopping high dose steroids cold turkey can send you into acute adrenal insufficiency, and that can be VERY dangerous.