Suggestions:
1) Estimate based on the size of the ferrules.
2) You don't have to tear down the bass to measure. Take one string off. Even keeping the tuner in, you prob'ly can pry the ferrule out of its hole while it's still in place. Measure hole, pop it back in position, keep playing until the new parts arrive.
I do not know Broncos, if they're all the same or what.
Edit: them being loose and rattling makes me hopeful that you'll be able to pop it off and get an accurate measure. GL
