Very odd. I don't pay for the $30 wifi hotspot feature so I cannot comment on how well that works (or doesn't work) after the upgrade. But the USB tethering definitely doesn't work after the upgrade. If you cannot use the hotspot feature on the 4G network then that should be something that Sprint supports. I would go into the store and have them look at it, or flash your phone back to 2.1 or something.
You could (if you really want to) try some instructions for unrooting a phone (which flashes it back to default -- and is old enough that I believe it's a 2.1 version), but why? You're paying Sprint $30/month to have a 3G/4G wifi hotspot and after the upgrade your 4G capabilities are broken. I would let them deal with fixing it!
I wouldn't personally recommend this but here is a link:
[How To] Unroot and return to stock. - Android Central Forums