Kudos on the weight loss and lowering your blood pressure.
How long have you been in this holding pattern? It's time to shake things up a bit to get the rest off.
I say look closely at your diet and maybe rethink your exercise routine. What exactly are you eating and what is your daily calorie intake? You may just not be eating enough and/or macro percentages may not be correct for your body type.
Your body probably just isn't conditioned enough to be running on the treadmill. Running on a treadmill is jarring. Stay with brisk walking on the treadmill. To be honest there is no need to run on a treadmill. Plus, brisk walking is easier on your joints.
Whatever you add I recommend something low impact until your weight comes down a bit more.
I've never experienced scrotum pain, but I have had pain around the buttocks. There are muscles there and they can be tight and can be weak. Tight muscles or inflammation in the area can pinch on nerves that run through that area as well. The periformis muscle can cause the rectal issues, as a form of sciatica, though that would usually run down the outside of the affected buttock and down the leg. Hip problems can be felt in the groin area. But I'm guessing from your description, given you're not running very far, that there are really two possibilities: the setting of the treadmill isn't working for your body. The best way to run is probably on dirt trails outside, but treadmills have incline settings and you could be overworking the muscles in the buttocks and glutes if you're doing inclines. You don't say what else you're doing, but maybe you're just a really stiff guy with little mobility -- not flexibility, but mobility -- in your joints. You can work on that. But when you mention the scrotum, unless you mean the groin, there's no muscles there and that would suggest, and I don't mean to be silly here, that whatever you're wearing to exercise in is too tight. Or whatever you're working in. This also could be caused by too much sitting -- you're used to being active and sitting puts pressure on that general area, especially if your posture is poor and you're leaning. I'd also ask, do you warm up before you run? Do you do any other form of exercise in that gym -- repetitive exercise on the same body parts can lead to injury.
Use the treadmill only for warming up; 5-10 mins. Drop the aerobic to 3x/week (elliptical and stairs) and increase the strength training 2x/week. Two days down. Take the calories down a notch, maybe 2200-2400 range. 2200=days down and 2400=days on. What's your macros like? Your fat, protein and carbs procentages? No processed stuff, alcohol or refined carbs.
Good news is my blood pressure went from 193/92 to 112/73 and my resting heart rate from 99-104 to 72-75