I'm in the same boat and wish I had a solution. Depending on my mood I'm using either the EViO 1.8 or Azreal 4.0 Sense ROMs, or I'm using the Blue Haze Gingerbread AOSP ROM. Regardless of ROM, Radio, NV, PRI, etc, I get uploads up to 1.4Mbps, but cannot get over 128Kbps downstream. Locked at very slow 1x (2G).
In my case, I believe that the radio in my phone has malfunctioned which really sucks because its never been dropped or even handled roughly. Of course I didn't buy Sprint's coverage (I never have) because I've never used it since there have been cell phones. Figures that the first time I buy a 'latest & greatest', it would be the one to go on the fritz.
Ever notice that cell phones are the only consumer electronic appliances sold that basically come with no warranty? Ever notice how strangely enough this has become acceptable to people?
Good luck, and I hope that you find a fix for your phone's issue!
The thing is, most sense rom's (including stock) will show that youre on 3G even if youre NOT on 3G, but rather on 1x.
CyanogenMod gives it to you straight This is not to say something isnt wrong (might be) but its a possibility.
Btw, all root related stuf in the roms/hacks section. Thanks.