have you lubed all the linkage in the wiper arm assembly and pushrods? that can sometimes make things sticky.

You have a loose/damaged connection between the switch and the wiper motor, it seems. The motor is gettin it's 12v, or it would not work on wash. The wiper motor works off of grounds, not voltage, which is backwards from normal. There is 12v on the motor anytime the key is on, and the switch grounds the different terminals to complete the circuit, and turn the motor on the different speeds. The washer part works off of different wires/connectors. Let me look at a diagram, and we'll see what we can find...

Otay...on your motor, there are two connectors. One has two wires, Yellow(12v+), and a dark blue(ground from switch) This is the washer connector.
The other connector, has three wires.
Yellow(12v+)
Black-ground from switch(high or low speed)
Light blue-ground from switch(high or low speed)
Your washer works, so we know we have 12v there, as the two yellow wires have the same source of 12v.(wiper fuse)
You need to ground the black wire at the motor(key on), and see if the wipers work, and then ground the dark blue wire, and see if it works. If the wiper works on the hi/low speeds by grounding the black or lt. blue wires, the motor is OK. You have either a bad switch, which you have replaced, the switch itself is not grounded,(but, again, the washer works) or a loose/corroded connection in the black/lt. blue wires from the switch.

Verify that there is 12v on the yellow wire(key on), in the three wire connector, first, just to be sure. If not, there is a break in the splice for the wiper motor(the yellow wire) Jump across the two yellow wires, in the two connectors then, and see if the wipers work. The yellow wire get it's voltage(power) from the wiper fuse, in the fusebox. The fuse is good, or the washer would not work. But there could still be a break in the splice, where the yellow wire "t's" off to the two connectors.

So, if you have 12v on the yellow wire, and ground the black OR the blue wire, the wipers should wipe.
Whew!! I know it's long winded, and may not make any sense, but, that's the way it works! Hope it helps. Adams' Apple38880.8395601852

Check that the switch is grounded. Mine did the same thing and like you said it is hard to drive and press the wiper button when it rains. My guage cluster was cracked below the switch so the top only grounded when switch was pressed. I installed a wire connecting behind switch to lower portion and it hasn't missed a lick. 

There ya go!!! Could be just the thing. Thanx for the info, Ray!!