Ceiling drywall - options for dealing with joists slightly higher than beam?
Easiest is to pack down each joist as you say with a furring strip.
It's slightly complicated if you say the gap varies, especially if it's from one end of the joist to the other, i.e. tapers. Then it's tight string line time between the beams and measure each furring strip and cut each to size.
But since it's a garage, you might get away with a 1/2&quot; strip everywhere if a slightly lower standard of absolute flatness is acceptable.