Alright, so I just bought a used 1996 Explorer for work. The truck is perfect, except I went down Deerfoot trail in Calgary the other day and after having it for two weeks, I noticed for the first time that the truck was making a bang sound and feeling like it was missing intermittently at 100km/h or over. If I were to guess it was almost like it was constantly trying to shift into another gear but it doesn't change RPM. When I pulled off of Deerfoot Trail, it seemed hesitant even down to 20 km/h to shift back into first. Did the same on the way home, but when you keep your speed down and are driving around town, it seems fine.

Tonight (trying to be proactive) I brought it into Canadian Tire and they did a transmission service (fluid, filter) and told me that it was fairly black (didn't look that black to me on the dipstick) and that there was a lot of black crap in the bottom. They proceeded to replace the fluid and filter and then told me it was only running 8-12 PSI when they figured the pump should be pushing between 20-40. I left and drove it to subway, then home and it seemed to be shifting a little harder and I never noticed the banging I heard before, but it wasn't that long of a road test.

The truck has 254,000kms on it so It could quite possibly need a tranny soon but any opinions here?

Only other information I can provide:

-Just replaced a DFPE (EGR pressure sensor) just before I noticed this
-I do have the common bouncing speedometer problem but only really bad between 25-45km/h Only moves 1-2 km/h at a constant speed above 50km/h. Changed speedo sensor, no luck. Thinking it's the gear in the tranny. Don't have a speedo cable on this one.
-4.0L Auto

Any help would be awesome guys!

thanks