If i overfilled my transmission fluid could it cause gear slippage
I do not think so but overfilling your transmission with transmission fluid can cause erratic shifting, starvation of oil and damage to the transmission. You can go to http://autos.yahoo.com/maintain/repairqa/transmission/ques123_0.html for more information. I hope that helps.
If u overfilled it why don't u just crawl underneath and drain some tranny oil out?
Yes it can
No but it will make a big mess