Before you decide it's the thrust block fighting you try loosening the nuts holding the other axle in & see if the axle currently giving you problems goes in...If it does than yeah you have to deal with the thrust block...But if it doesn't the most likely you have a slight misalignment of the splines in the side gear & the cone...My picture below is of a clutch but cones have a similar spline design... If thats the problem tighten the nuts on the first axle then loosen the nuts on the problem axle slightly & mount a wheel with a couple lugs, you don't need the brake drum... Now grab the wheel & try twisting/rocking the wheel while pushing inward...You should feel the axle move inward if/when the splines align...

FWIW my understanding is with the clutch style you need to remove the thrust pins but the cone style the block doesn't need to be removed...