hello. IF the transmission was driven with a leak far enough (2-5 miles) it could have damaged the 3-4 clutches. these are the "weakest point" in the chevrolet transmissions. the best way to tell IF the clutches are damaged is to remove the transmission pan and look for a "black grit/material" in the transmission pan or filter.
IF there is a substance found in the pan... the most likely cause is the clutches... and the only "fix" would be to have the transmission overhauled/replaced.
i've built hundreds of GM transmissions at my dealership... and 90% of the failures are 3-4 clutches... due to fatigue, slippage, or towing more than vehicle capacity.
hello. removing the pan would give you the best overall idea of the shape the transmission is in. IF there is alot of metal, clutch material, or rubber pieces (from o-rings inside the trans) in the pan... an overhaul would be your best bet.
as for the filter being clogged up... i've had several units with what amounted to sludge in the filter and the transmission still had first gear! the filters have alot of surface area inside them.
it could save you a diagnostic fee to remove the pan and inspect it yourself before sending it to a shop... but since you've lost 3rd and 4th gears... the most common issue is clutch failure.