Do dwellers heal in the wasteland in Fallout Shelter?

No, it's not possible for dwellers to heal by themselves in the wasteland. However, dwellers heal all damage when they level up in the wasteland.

But I don't really know how much health they have, might be important she's level 36, has 7-S, 1-P, 10-E, 6-C, 6-I, 4-A, 4-L.

You said it yourself, she is not level 50. There you go. And because she has 10 Endurance, she hardly takes any radiation damage.

I try to max dweller stats before I send them out to level, so I have a level 8 dweller with 25 stimpacks and high +E gear and a big gun take about 2 days before they use the first stimpack. That's because with every level up their health and radiation will reset. In addition, if they have more than 10 Endurance, they will never take radiation damage (red bar).

I didn't have an undammaged dweller right now, but I think it's obvious that this guy would not survive 4 days with only 6 stimpacks without having some additional healing going on. I believe he was level 8 when I sent him out.