OK, if you're using QPST to change your settings from Mob IP to simple IP, you're halfway there...
Go to the M .IP tab and PPP config tab, look for (yournumber)@aaak.mymetropcs.com and change it to read (yournumber)@mymetropcs.com. You'll also need to do this under the Um and An tabs, in the PPP config. section. Change only those settings though.
I had edited my build.prop but was unable to tether until I changed those settings. To be clear, you're removing the .aaak portion only. I believe that's the method they use to hide something from us.