Slipping trans?

If a worn band, I think it would affect both 1st and reverse. However, even then it may not be the band, but the servo piston that applies the band. One piston is known to often **** in its bore (forgot which) and there is an inexpensive after-market upgrade. I think you can get at all without removing the valve body, but you do have to drop the pan. Get at least a shop manual first and maybe a trans manual. You can easily apply air to test the pistons if you have an air compressor, if you remove the valve body.

If it is the lip seal that others mention, that requires removal and rebuild. I wonder if it could be symptoms of a worn front pump. That, and many other things can be tested by measuring pressures on the ports outside the tranny. You might do that first or have an experienced shop do so before you pull the pan.