Tranny Slugish

I have a Econoline 3/4 ton van that has been sitting for a number of years and I can start it okay, but the tranny seems to be slipping. Is there anything that I can add to the fluid to solve this problem?

Try using Trans-X. It is a transmission oil additive that helps with tranny slippage, leaks, etc... I bought this at O'Reilly's Auto Parts.