To get the smoke opacity down, a good run prior to the test should help.
You havnt stated what make and model of jeep it is.
I'm assuming it's an older vehicle?
If a good run doesn't sort it, I'd be speaking with a diesel mechanic before using any additives.
It's could be down to a faulty fuel pump or bad injectors.
Basically roast the engine before bringing it for the test. If it still fails on smoke, get it looked at. But I'd spend nothing first. I wouldn't even put in an additive as it can mask an issue.