My Daughters '90 D150 had a similar problem a while back  here are the
notes I posted to the newsgroup Oct 3 2004 (BTW they are still
available on Google Groups.) under the subject
UpDate: 1990 D-150 WS Wiper Question

For any one that wants to know for future reference.
1. Remove wiper arms. Lift blade arm away from window, rotate keeper
up let arm down on keeper, wiggle assembly off rotator assembly, Set
aside.
2. Loosen screws under windshield weather-strip.
3. Remove screws in engine compartment.
4. Loosen cowl Grill pull out far enough to get windshield washer
hoses off of the nozzles. Remove cowl grill.
5. Remove nut holding arm to wiper motor. Try not to move motor
shaft.
6. Remove three bolts holding each wiper pivots to body
7. Note the way the arms are positioned in relation to the other
parts (this is so you get the new parts installed in the correct
direction.
8. Remove all the parts to the work bench. (Do not try to do the
following in the truck it is not worth the hassle.)
9. Remove old bushings if necessary.
10. Install new bushings by pressing into arm with vise or pair of
large channel locks. (I used a small bench vice 11/16 socket as
backer to keep from bending the arm as I pushed the nylon bushing in
to the arm and on to the pin).
11. Assemble arms onto pins.
12. Slip entire assembly back into vehicle.
13. Reinstall both wiper pivots (do not tighten bolts yet)
14. Install drive arm to wiper motor.
15. Tighten wiper pivot bolts.
16. Test operation. Correct any problems.
17. Reinstall cowl grill.
18. Reinstall wiper arms. With blade about 1 1/4 inch from bottom of
windshield.
19. Test wipers for full range of operation should not hit drivers
side A pillar when at top of stroke. (You may need to remove arms and
reinstall them in another notch on the pivot to get every thing
right).

Happy Camping. ldpoos@NOPANTS.juno.com
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2003 Dodge 1500 QC SB Hemi, A/T, Tow Package, 3.92 gears (11 mpg
Towing)
2001 Aerolite 21RDB 21ft TT (Scales 2900 dry)