Yup! That's the Y_axis. OK, There's a possibility that your frame is off the square causing the bearings to bind on the straight rods.
I'm assuming the only difference you have from me (I've got a single extruder B model) is the second extruder.
Can you slide one of the straight rods out of the back of the printer so the Y bed is only supported by the other one. Move it back and forwards. Does it still feel stiff? If it does, you'll have to check each bearing on that side independently which unfortunately means snipping the ty-wrap/zip fastener.
However... If it's fine and free with one side disconnected, check it with the other rod replaced and the first one removed.
At this point you've possibly found a bad bearing or that each side on its own is free.
If each side is free but you get binding when both straight rods are fitted, I'd reckon the base of the printer; front plates, back plates M10 screwed rods and straight rods are not square.
Can you measure the distance between the Y straight rods at the front and back of the printer? Because you say it's tight all the way, the measurement should be the same. Can you check the distance between the associated linear bearings (left one to right one, front and back. i.e. are they aligned properly.)
If the back and front plates aren't at 90 degrees to the M10 screwed rods, the distance between straight rods would be reduced enough to "pinch" the associated linear bearings on the Y carriage bed. Can you check this? I just used a steel ruler to check the distance from the M10 screwed rod end to the end plate was the same,which seemed to work. If there was an appreciable difference you'd be off square. Better idea. - Maybe you can find a kid's geometry set-square to check for 90 degrees between the rods and end plates?
All that's left that I can think of is the alignment of the four bearings in the Y bed carriage, but you'd have found something wrong earlier in the tests probably.
Anyway... Check that little lot and report back.
Good luck!
Play Bonny!
Soadyheid