I just spent 3 days researching this issue, finally got it fixed. Same thing that happened to you. I am assuming your bootloader is unlocked but S-ON.
Tings you will need:
Android SDK
HTC Drivers
7-zip
RUU_Speedy_Sprint_WWE_1.17.651.1_Radio_1.07.00.1129_NV_SPCS_1.52_1103_release_160079_signed.exe
On your computer, search for ROM.ZIP. Include system locations and hidden files.
Delete any instances of it that you may find.
Take the SD card out of the phone.
Put your Shift into fastboot (vol/down + power)
Run the RUU, let it get stuck.
On your computer, run a search for ROM.ZIP while the RUU is running. Its gonna show up in some temp folder.
Copy it to your desktop.
Use 7zip to extract it (for whatever reasons Windows can't extract it).
Open the extracted rom.zip folder.
Copy all files and place them in your SDKs platform tools folder.
Shut down the RUU, reboot the phone into recovery (vol/down + power).
Run CMD and change directories to your platform tools folder.
Type the command "fastboot flash recovery recovery.img" (without the quotes) - this gets rid of any custom recovery like Clockwork, etc.
Reboot the phone into recovery again, you should have a triangle with an ! next to it on black background.
Go back to CMD and your platform tools, and start flashing the following:
fastboot flash boot boot.img
fastboot flash dzdata dzdata.img
fastboot flash partition partition.img
fastboot flash radio radio.img
fastboot flash recovery recovery.img
fastboot flash system system.img
Reboot the phone.
After 30 seconds of white HTC screen, your phone should boot up fine and you will be in Android 2.2.
Let me know if it worked out for you, I just fixed mine less than an hour before writing this up.
Also, if anyone out there sees anything redundant, or can help me clean this process up a bit, I welcome your input.