Ballpeen,
You said this started all of a sudden. On that note,,
Crawl under and check to make sure the KD lever is moving all the way forward to its resting position.
If it is, then most likely it is being caused by a sticking valve in the VB, or a stuck/broken check ball in the VB.
Normally a plugged filter will not hold back a shift but will cause it to run-away during a shift.
If you have the knowhow and a manual pull the VB and go through it. Check for stuck valves and balls. Also check the accumulator valve (above VB in the case)A real plus would be if you have a straight edge to check it for warping because if you have ever overheated it, the VB takes the brunt of the heat.
90% of all shift problems I have encountered eminate from the VB.
Dave