Transmission not shifting

My vehicle will not shift after first gear. It will go about twenty to twenty five mph and then the truck will not go any faster, the rpm's only raise and does not shift gears. I had changed my transmission fluid and filter three weeks ago and the truck ran great for three weeks after. I noticed it was leaking a small amount of transmission fluid from toward the front of the transmission. It started shifting hard when I took off from a stop first to second gear when the fluid was low, filled up with fluid and ran great. Yesterday started shifting hard again. Today I checked the transmission fluid and topped it off, now the truck will not shift higher than first gear and when the speedometer gets to like twenty mph the rpm's only raise and not shifting. Could the leak on the front pan make it not have pressure to shift gears or could this issue be something else? The front pan seal is leaking, starts with a drip, but pours out heavier when the gears are shifted.