Will 3.23 gears, from a 98 z-28, fit in my 86 T/A? (factory 2.73 gears)

yes they're from the same 7.5 ten bolt, jsut watch and make sure the carriers are the same series and if you change carriers make sure the axle spline are the same.

Right, the gears will work, but not on your 2-series carrier. The late-model carrier will work, but not with your 86 model 26-spline axles.

what do you mean series?

2-series = the carrier for ratios 3.08 and lower (3.08, 2.93, 2.73, etc.), 3-series = the carrier for ratios 3.23 and higher (3.23, 3.42, 3.73, 4.10)