How do I introduce to other dogs

My dog Svea came had a wild past life. She was loose running around and had puppies while on her own. She's a dream with people and kids and listens very obediently whenever misbehaving.
But my little gem has such a hard time making dog friends, especially when she's on a leash (and she's always on a leash). When a dog barks at her, there is no way they will meet (if she's leashed) and she will actually want to make friends. She will stand tall and sniff them, but I can see the hair on her back raised and eventually she'll do a half bark with a small jump (still on all fours) to test them. This is where I stop the introduction, pull her back, and we walk along. I don't know exactly how to introduce dogs; and I am not about to have a dog fight on my hands.
I've never seen her be aggressive other than in that situation, and I'm not sure if she would overcome those issues after a little time with the dogs since I've just never wanted to find out. I know when she's off leash she's much easier and laid back.
Sometimes when she's leashed, if the dog approaching her is happy or not too concerned with her, they sniff each other and move along.
Interestingly as well, even the dog friends she does have, she doesn't instigate play time. She will only play with them if they start things up first.
Any advice on this?