if you disconnect just the small belt that goes between the rear rod & the motor, how does it feel?
it is possible that the left & right carriages are misaligned, causing slight binding...
to correct - loosen the set screws on the pulleys on the rear rod - work carriages back and forth until it moves smoothly, set carriages near the middle of travel on the Y rods in a neutral position and re-tighten the pulley setscrews being careful to not twist the X axis.
video here: http://wiki.solidoodle.com/belt-adjustment
SD4 #1 & #2 - Lawsy carriages, E3D v6, Rumba controller board, mirror bed plate, X motor fan, upgraded PSU & Mica bed heater
SD4 #3 - in the works ~ Folgertech FT-5, rev 1
Printit Industries Beta Tester - Horizon H1