'99 DII with 125k, i've owned it since 90k. Thinking about doing a tranny service soon. I'm unsure if the 30k tranny service was done. Should I assume that it was done, and just drain and refill? Or, should I do the complete flush, filter, and gaskets/o-rings?

 

should not flush a tranny with such high mileage. Remove drain plug and allow fluid in pan to drain, remove pan and clean out sump, replace filter and reinstall pan. fill trans with about 2qts atf and 1 bottle lucas additive. start vehicle allow to warm, run shifter through all gears a couple of times do not move vehicle, remove fill plug from tranny and top off with atf.

If you do a search I remember seeing a more detailed descriptionseveral months ago. but do not flush the tranny, chance dislodging large gunk that will possible kill your trans.

 

So, replace filter, all gaskets and o-rings (found the kit at autozone for about $40).no flush.
Approximately how many quarts will this take, i already have the lucas.

 

Since you have no history on this truck, I would replace the filter, and get in the habit of doing it every 20,000 miles instead of every 30,000, it's not new any more and qa little extra attention will go along way.

 

10-4, done.