95 E250 Cargo 4.9L I6 questions

I just purchased a 3/4 ton 1995 E-250 Cargo van with I6 4.9L engine and 3-speed (I was told) auto transmission. A few questions/problems:
What is the transmission model for this vehicle?
Van drives smooth until I get on the highway and approach 55-65 mph. Once I get into this speed (I'm thinking) it's in the upper range of 2nd gear. A loud low frequency hum then starts which gets a bit louder until I accelerate around over 65mph and then (again I'm just thinking) it shifts into 3rd gear and goes back to sounding smooth and normal. The vehicle doesn't seem to lag or slip at all.. it's just this loud deep almost rumbling sound... definitely not shaking or anything involving movement of the vehicle. Could this be a delay in shifting leading to high rpms at the upper end of 2nd gear? The vehicle has been sitting in a car lot for a long period of time and I was thinking perhaps may just need tranny filter/fluid change. Then again it could be something else in the drivetrain, I'm no expert. Any recommendations as to my course of action here? Also any recommendations for brand of fluid to use? Is there an easy way for me to perform the filter/fluid change without the old fluid spilling out in all directions when i drop the pan?
This may or may not be normal for a large cargo van but it seems to me there is a large amount of inertia when accelerating this vehicle almost as though something is dragging.. even when coasting downhills the van is decelerating and slowing down. New front brake pads were just installed @ the shop. Could be something in the drivetrain though I wouldn't know what areas to look at. Again.. any recommendations for things to check?
Please keep in mind I'm barely an amateur when it comes to auto repair.. I've worked on a '93 Taurus 3.0gl and an '86 toyota tercel awhile back.
Please do not reply via email, instead reply in this newsgroup to minimize my spam problems. Thanks for your help!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.