The submitted crash is in ALTQ, you might try removing the traffic shaper and upgrading and see if it's stable that way.
db:0:kdb.enter.default> bt
Tracing pid 12 tid 100006 td 0xfffff8000396b000
kdb_enter() at kdb_enter+0x3b/frame 0xfffffe00f439f820
vpanic() at vpanic+0x19f/frame 0xfffffe00f439f8a0
panic() at panic+0x43/frame 0xfffffe00f439f900
hfsc_dequeue() at hfsc_dequeue+0x633/frame 0xfffffe00f439f940
tbr_dequeue() at tbr_dequeue+0xdf/frame 0xfffffe00f439f990
vlan_start() at vlan_start+0x12c/frame 0xfffffe00f439f9f0
tbr_timeout() at tbr_timeout+0x57/frame 0xfffffe00f439fa20
softclock_call_cc() at softclock_call_cc+0x18a/frame 0xfffffe00f439faf0
softclock() at softclock+0x94/frame 0xfffffe00f439fb20
intr_event_execute_handlers() at intr_event_execute_handlers+0x20f/frame 0xfffffe00f439fb60
ithread_loop() at ithread_loop+0xc6/frame 0xfffffe00f439fbb0
fork_exit() at fork_exit+0x85/frame 0xfffffe00f439fbf0
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe00f439fbf0
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---
That said, I'm running 2.4 on an APU (first gen, not APU2), with traffic shaping on and it doesn't crash there. I'm using PRIQ though, not HFSC.