A 97 could have the 6.8 - that was the first year for that.
It is very possible that you're getting misfires without codes. That chassis was not required to be OBDII compliant, even though it has all of the OBDII equipment on-board. The things (like a misfire) that passenger vehicles from '96 forward that are required to set a MIL for are recorded and noted by the computer in the medium-duty chassis, but the MIL stays dark and no OBDII codes are set.
The symptoms you describe sound very much like a bad coil to me. It could be a transmission issue, but given the mileage and the prevalence of CoP failures as well as the ease of diagnosis, my recommendation would be to assume you've got a misfire. You need some sort of OBDII diagnostic interface that has the capability of running test $53. You can get a Bluetooth OBDII adapter and the Torque Pro software for your phone or tablet for very little money or you can get a good adapter plus a laptop and ForScan which is el-freebo. Personally, I like Torque Pro for driving around and I wouldn't own a Ford or Mazda without also having a good ForScan setup - but either one will work for our purposes now.
Assuming that you go for Torque Pro, get everything set up and installed, plug in the adapter, get the motor running, and from the main Torque Pro screen select "test results". It's a little bit slow, but eventually you'll be able to scroll down to find test $53. $53 is your test number and that test will have several CIDs indicating the cylinders. Those numbers are hexadecimal so 0a is equal to 10. You'll see that each CID has a Max and Current number. Ignore the Max - current is showing the number of misfire events for each CID or cylinder. You want to look in that list to see if there are any CIDs that have a large number in that "current" section.
When you do, you can prove out that it is the coil that is the problem by grabbing yourself a 7mm socket and ratchet (1/4" drive is fine) and swapping the offending coil with another one that does not show a large number of misfires. Your cylinders are numbered thusly:
FRONT OF VEHICLE
6 1
7 2
8 3
9 4
10 5
---------------------
TRANSMISSION
Coils are super simple to swap. Carefully undo the clip on the wiring harness to release the connector. Do not break the tabs. If it's cold out, hit the connector with a heat gun or hair dryer to warm up the plastic and take some of the brittleness out of it. If you break the tab it will be very difficult to keep the coil plugged in in the future, so be careful. Once the wiring is disconnected, remove the 7mm bolt (lefty-loosey) and then just pull the coil up off the plug. It should come out with its boot. Those boots are separate and can crack and also cause misfires. Inspect the boot very carefully.
Pull your coil from another (non-misfiring) cylinder in the same way and then carefully swap the boots - the only thing we want to change is the coil! Again, use heat to make everything a little more pliable and less likely to tear, crack, or break. Re-install, make sure everything is connected and the coils are bolted down - don't over-tighten the coil bolt, it's just there to keep the coil from falling out.
Start the engine up, go back to Torque Pro, reset all the counters and watch test $53 for misfires again. You should see that the misfires have moved to the cylinder with the bad coil. If that's the case, grab two new coils (they usually come with new boots). I like Bosch and Motorcraft - stay away from the no-name junk at your FLAPS. Replace our tested bad coil with one of the new ones and put the other new one along with a 7mm socket and ratchet handle in the glovebox for next time. And there will be a next time. They just do that.
It is possible that when you perform the test, the coil isn't bad enough to cause misfires at idle yet. You may need to get out and drive the rig around a bit to cause it to misfire so you can find the offending cylinder. If you do the test in Torque Pro (or whatever) and even after driving it around and getting it to exhibit the behavior you don't see any misfires, then I'd dump the trans fluid and do new fluid and filter.