1999 E250
4.2l
4R70W/4R75W according to Fordparts.com
170,000 miles

I've owned this van for a year, but have put less than 1000 miles on it. I purchased it thinking I'd need it for business, but haven't needed it and so I want to sell it. I do not know the history of the vehicle.

Cleaned the EGR intake ports last month and got rid of my P0303 codes.
Installed new plugs at that time, but not new wires. I've had no codes since then.

The tranny is shuddering under moderate exceleration at speeds from 40 to 65. I've heard this can be the tranny, the torque converter, or coils gone bad (among other things).

I've not changed the tranny fluid. I have added Lucas Transmission Fix, which didn't seem to help.

How do I test the coil, or do I just replace it and the wires and see if that works? I'd rather save the money if the coil is fine, but if it's the coil and I can get this thing shifting smoothly I'll get my best sale price, so I'm willing to spend some cash if I must.