Altium Differential Pair Routing Via to Via Clearance
I would use the "InDifferentialPairClass" and "IsVia" clauses in Altium:



I haven't tested this, but theoretically if the object is a via and is in a differential pair, then it won't complain unless the clearance is less than 0.15mm.

Just make sure this rule has a higher priority than the original clearance rule (which is currently being broken).