What does it do when you manually upshift it?
Did you say it slips in 2nd gear or 1st? Between gears it's hard to detect a slip.
If it actually was slipping the Fwd clutches or the Band, I would go buy a filter and pan gasket, remove the pan and check for debris. If it's clean in the pan, put it back together with a new filter and try to adjust the modulator. If there is a lot of metal shavings and friction material, your problems are just beginning and if you continue to drive it without rebuilding it you will damage it worse.
Note: there is a tab on the side of the filter that retains a spring and valve, make sure it gets put back together with the valve cup pointing towards the valve body and the spring goes in last before you install the filter. Some technicians loose this little valve and replace it with a 1/4" steel ball bearing, which will work also as long as it goes in the well first and the spring is installed last. ~BH
