The 34th VA Cavalry Battalion was in Jenkins Brigade with the ANV in the Gettysburg Campaign. They are listed in the OOB of that campaign in every source I have. You cited an 1862 note from Witcher. Not the same thing.
I went through the OR last night for the August-November 1864 Shenandoah Valley time frame and Witcher does sign a report dated October 5th, 1864 from Greenbrier County, West VA. But that does not mean he did not make it back to Lomax's Division, of whom he was organic, in time for Tom's Brook. There was a lot of moving cavalry units around on the Confederate side as brigades were formed and re-formed as needed and based on who needed what troops to hold the central valley and the upper valley near Abingdon. In fact, it is almost impossible to tell who was where. The later October returns for the Valley Army in the OR show the 34th VA Cavalry Battalion was with Lomax as stated. That may be the OR report Wittenberg used.
The 37th VA Cavalry Battalion may have been involved at Tom's Brook as it was sometimes in Johnson's Brigade. Johnson also had the 22nd VA Cavalry regiment there.