[Keith Seymore]

I'll take a shot:

ASM = abbreviation for "assembly"
PRIME = don't know, but it appears that if it is an "unusual" feature then engineering had to tell them to prime/paint it (if that's the case then it apparently was also available "unprimed", and there would be a code for that to balance the release out
E6D = could be a country code for Gaza, or perhaps that's just a reference from a different time period from when yours was built. RPO codes do get reused so it could me one thing for a model year and something else for a different model year. For example, my book from 1998 does not show an E6D but it does show an E6E, which is a speedo adapter sleeve. Yours could be in that same family of RPOs.
E8A = same as above.
GQ1 = just means base (not optional) axle. Ring gear size depends on the model series of the truck.
MX1 = a merchandising option is one that functions as a "placeholder" until the specific "purchasable" option can be worked out. For example, you might see "MM4" as a merchandising option for a manual trans, and then the specific manual trans to follow (ie, M20, M62, M64 - particularly when the specific trans will vary based on other option content, as is the case with M62/M64 - same trans but different low gear ratios).
K19 - Air injection - is supplemental air shot into the exhaust for emissions purposes (like a California emissions package)
UP7 - radio, manaural - should mean "one speaker"
Letters after the tire typically designate load range, tire type (ie, all season, on/off road, sidewall type - black wall vs raised white letter or white wall) and other less interesting junk
YD3 - you dont' have a front axle? What's keeping the front bumper from dragging on the ground? (I'm kidding of course). That designates the weight capability of the front corners (springs, knuckles) etc which is considered the front axle, even though it is an independent front susp and not a beam type axle
YD6 - I'd say those are the base springs. There are separate RPOs (at least two of them) specifically designating HD springs.

You might also notice that "X" represents front tire, "Y" for rear tire and "Z" for spare, followed by the specific tire designation "YN".

K