How likely is it for a 91B to be stationed on a TRADOC base?

Do you realize that a 91B can be stationed anywhere? Out of all the bases in the Army, you are asking about MAYBE 3% (and I think I'm being overly generous) of bases that has TRADOC. You could be stationed from FT Irwin to FT Drum. If you're hoping for a TRADOC base only, you're in for a bad day.

Depends on the base, but pretty damn low. Bases that don't use a lot of military vehicles tend to hire civilians for the mechanics

In my experience unless you’re in an instructor position no too likely.