Have I missed anything before taking it to a shop?
1994 E4OD got the factory warranty upgrade the first year.

After transmission warm up the shifting is erratic. Shift from 1 straight to 4. Around 50mph in OD it only goes into 4 with light throttle but will not hold 4 more than a couple of seconds then back to 3. If I use very heavy throttle while it is in 4 it will hold it to 75 but when I let off it shutters then slams between 3-4.

At 170K miles replaced the filter and drained the TC. Filled with Mercron.
Then at 19OK and 65mph it suddenly slammed into 3 then back to 4 every few seconds. Only code was 622 KOEO circuit fault #2 shift solenoid.

Since then I have replaced the solenoid pack twice, VSS, MPLS cleaned all the battery cable connections. I even got the transmission wiring harness on the table and every pin tests perfect. Wiggle tested the #2 wire. Could not find Mercron on the solenoid pack replacements so went with the D??/Merc. Continue to get only code 622 after clearing them. Found pan was shiney and clean when solenoid pack was replaced and the transmission had worked perfectly until now.

So have I done every thing possible before going to a shop?

What does it sound like to you ?