94 Chevy Astro Transmission Problem

My 94 Astro has over 155,000 miles on it, about a month ago when I was merging onto the interstate I noticed a large cloud of smoke behind the vehicle. I pulled over and the smoke was coming from transmission fluid on the exhaust. The fluid level was about 1.5 quarts down, I added fluid and looked for leaks. At idle it didn't leak, I found that the transmission only seemed to leak above 70 miles an hour(I don't have a tach on this van so can't give an rpm). I checked the outside of the transmission and couldn't find a noticeable leak point, it looked like it might be coming from the front of the pan and spraying onto the exhaust, so I changed the pan gasket and filter. Test drives looked good and I a thought the problem was fixed but it occurred again(at the same merge point onto the interstate). This happened after a 75 mile drive at speeds between 45-70 miles an hour on side roads, no indication of leakage, then when returning at less than a mile from the entrance ramp to the interstate smoke was noticeable, transmission fluid on the cross pipe, pan, muffler, and rear differential, and leakage on the ground which stopped while sitting with the vehicle at idle.
If anyone has seen this, I can't tell if it is coming from the seep hole on the torque converter cover , but I keep thinking that if the front seal was cut or nicked it would leak more than this, since changing the fluid and filter 2 weeks have passed and I have driven the van back and forth to work daily average speed 55 miles an hour 20 miles one way, with no signs of leakage.
Thanks in advance for any help
Dan Hi;

This is what can happen when changing the transmission fluid in a high milage transmission if it's never been serviced before.

I would suspect that either your front seal or your front pump cover gasket on your transmission are bad. This is what you should check into.

If it's not leaking real bad, you might get away with putting a trans sealer Car Repair Talk's forum.