I had this issue last night, easy fix, posting for anybody else who may have done it.

rear shock/axle resistance video:
http://www.youtube.com/watch?v=oFobXeps080

at first i was sure it was a bearing issue, but after closer inspection i noticed the axle/universal joint was rubbing against the rear a-arm. This was because I had previously moved the bottom shock screw(s) to the center-most positions.

This can be fixed by one of 2 ways I figure, I put the screws on the outside position instead of the inside(center), giving the shocks a more "straddled" layout, and putting less pressure on the axle/a-arm. The second way to do this would be some minor jangafication, by boring out the inside of the bottom X on the rear a-arms, it would let you keep your height (shocks on center most positions) and remove the rubbing contact between the arm and u-joint.

Now when I free spin the rear tire's, with shocks extended or compressed, they spin freely for atleast 15 to 20 seconds with no resistance.