Q: 96 mustang automatic will go in reverse but not drive the rear seal was replaced what could be wrong

Sometimes it seems like doing repairs on older cars is like opening Pandora’s box. Failure of the transmission shouldn’t have anything to do with replacement of the rear seal, but it seems it has. It could be that simply draining and filling the fluid in the transmission may have been enough. with old transmissions, new fluid sometimes has the effect of breaking loose deposits that can then block the tiny orifices in the valve body, causing failures. You can’t blame anybody for this kind of failure, if it didn’t happen now, it would have down the road. There might be some other explanation, but Not much I can give you from this desk. Your need to have this thing looked at. Contact YourMechanic to have a technician come out to your home or office to check out your transmission problem and advise you as to the next best steps.