Are you adding it to the cure or the rub; or both? Believe it or not here are the substitutions for juniper berries.
Juniper Berries Substitutes
1 teaspoon gin for every 2 berries
equal parts crushed bay leaves and caraway seeds.
I haven't tried the above substitutions, but the equal parts bay leaves or caraway seeds must refer to volume measurement. I like adding juniper berries to the cure, it kicks it up to another level and makes the final product a deeper red, but I generally don't use then in the rub, because they can tend to over power the other ingredients. You can use just the spices you like, but for me cracked black pepper and coriander are two main essential ingredient for pastrami.
First post read equal amounts of bay leaves; or caraway seeds; which is incorrect.