"The printer will print perfectly on both sides if the paper is fed into the sheet feeder. If the paper is fed from either of the paper cassettes, the paper not picked up from the cassette. There is a mark on the paper where it appears the roller has been rubbing on the paper."
(From: Al Savage (firstname.lastname@example.org).)
Piece of cake. Replace the pickup rollers. You can clean them, but it won't last.
Now, the bad news: it's not real easy. Or, like most things, it's really easy once you've done it a couple of dozen times.
The rollers themselves are about $5 these days, and have been redesigned to eliminate one problem they used to have: you can't put them on backwards anymore!
LaserJet IIID: remove all exterior plastic: three screws up top accessed after opening the lid. One screw horizontally near AC power cord receptacle. Release nasty catch of top lid from side plastic at just above AC receptacle. Try hard not to break top lid while doing this. :)
Now you can access two screws retaining front edge of side panels to front panel. Remove two vertical screws holding left side panel to rear of printer chassis, near fuser.
Unhook front panel operator panel cable, remove front panel assy.
Remove six small screws retaining front chassis subframe to (scanner, HVPS, DC PS), plus about five larger screws holding power envelope feeder subframe to front.
Remove DCPS on right front of chassis: remove three vertical large screws (two in front, one at right rear). Disconnect main motor harness from DCPS. Disconnect gray fibre optic cable from black clip at left front edge of DCPS, or it will be damaged, and you'll add another large batch of screws to remove/replace! Then remove DCPS by grasping firmly and pulling straight up. This takes considerable force, as there are connectors at the bottom that you are separating.
I recommend removing the registration assy prior to removing the paper pickup assy, but it can be done without. To remove the registration assembly, remove four vertical screws (three black, one chrome, most likely on III series) from reg assy, which is the part with the green "handle" just inside the paper path, as you look straight down into the printer. The left rear (shiny) screw will likely have a short ground wire under it on a III. With the DCPS out, the reg assy will just lift straight out. Note carefully the bronze ground lug at the left front of the reg. assy. It must be rotated forward at reassembly or the forward black roller (small diameter, 9" long) on the reg assy won't be grounded, and you'll get all kinds of odd print quality problems. Look at it now, BEFORE removing the reg assy, to see how it's supposed to look.
Using long #2 Phillips, remove two small screws retaining the pickup assy (the shaft with the grey PU roller on it, plus the black or white spring clutch on the right end of the shaft) black plastic bearing blocks. Lift up PU assy, slide 2" to right (the reason you had to remove the DCPS), then swing left end rearward to where the reg assy used to be, and remove.
To replace the PU roll: remove E clip on left end of shaft, slide off all the washers, spacers, etc, noting the order carefully. Roll just falls off!
On reassembly, don't forget to reconnect the main motor's wire harness to the DSPS. One of my common problems.
If you're going to do the lower PU roll, do it now, prior to reassembling the top half. Remove the lower side plastic panels -- one horizontal screw in back for each side.
After removing the sides, remove the E clip on the left side of the lower pickup assy, then remove the bronze shaft bushing. Don't lose it.
On the right side, remove the three small horizontal screws on the clutch plate, disconnect the clutch's harness from the chassis harness and harness hold-down clip, then rotate the clutch plate and slide the lower PU assy out the right side.
Replacing the PU roll is identical to doing the top one at this point.
Now you know why I charge 1.5 hours to do top & bottom rolls. Plus, I pull the lower pan and vacuum it out, Fedron the reg assy, and replace upper/lower fans if needed.
The IIID is the best laser printer HP ever made, barring the Si series. Well worth the effort and expense, even if you have to replace the fuser too. I like them a lot.
Done right, a proper PM of a IID or IIID involves removing the upper unit from the lower to properly clean the skewed feed roller, but that's another 30 minutes, plus cleaning the refeed assy. You can "kinda" clean that lower feed roller (not the PU roller) if you have small hands and a long Q-tip, but it really isn't a good job.
PS: oh, yeah. I wrote this all from memory, and I haven't done one in a couple of weeks, so I just MIGHT have overlooked something. But the major steps are there. Keep track of the various screws, they're not all interchangeable.