I am using a cheap Trendnet TU2-ET100 (HW v.4.0R), and it has been perfect under 2.2, and is supported by the axe driver.  The interface is presented as ue0. There is one problem.  While the axe driver supports altq, the ue driver does not.

I found a useful post on the pfSense mailing list which offers a partial solution: https://lists.pfsense.org/pipermail/list/2014-March/005652.html

By editing /etc/inc/interfaces.inc and adding "ue" to the list of drivers supporting altq, the traffic shaper may be used with my USB NIC.  Unfortunately, this is an ephemeral change that will likely be overwritten when pfSense is upgraded.  If anyone is aware of a way to make this change stick through an upgrade, I would love to know.

After the edit, the pertinent section will read as follows:

function is_altq_capable($int) {
/* Per:

• http://www.freebsd.org/cgi/man.cgi?query=altq&apropos=0&sektion=0&manpath=FreeBSD+8.3-RELEASE&arch=default&format=html
• Only the following drivers have ALTQ support
  */
  $capable = array("ae", "age", "alc", "ale", "an", "ath", "aue", "axe", "awi", "bce",
  "bfe", "bge", "bridge", "cas", "dc", "de", "ed", "em", "ep", "epair", "et", "fxp", "gem",
  "hme", "hn", "igb", "ipw", "iwi", "ixgbe", "jme", "le", "lem", "msk", "mxge", "my", "nfe",
  "nge", "npe", "nve", "ral", "re", "rl", "rum", "run", "bwn", "sf", "sge", "sis", "sk",
  "ste", "stge", "ti", "txp", "udav", "ural", "vge", "vmx", "vr", "vte", "wi", "xl",
  "ndis", "tun", "ovpns", "ovpnc", "vlan", "pppoe", "pptp", "ng",
  "l2tp", "ppp", "vtnet", "ue");

Hopefully this post will save other users some aggravation, as the information was not too easy to find.  I have tried to salt this post with enough search terms to lead an interested user to this solution.