I am far from an expert on the subject, but perhaps I can provide a few clues for you to pursue.
From information in the letters we can probably infer that your uncle was involved in the bombing of Vietnam by B52 bombers. During the time frame of the letters (1966/1967) the B52 bombers were flying out of only two airfields. These were Anderson Field on the island of Guam, and U-Tapao airfield in Thailand. I don't believe that B52 missions were flown out of Thailand until early 1967, so during 1966 he was probabby stationed on Guam. Also APO San Francisco 96334 was the APO for Guam -- but perhaps other areas also. The Mu Gia Pass mentioned in the letter is a pass near the border of North Vietnam and Laos through which the Ho Chi Minh trail passed. The Ho Chi Minh trail and consequentially this pass became a common target of B52 bombing missions throughout the war.
I don't know what CRM#2 in the mailing address represents, but 322 BS Crew S-22. is the 322nd Bomb Squadron, and the crew number. My guess would be the 322nd was probably part of the 4133rd Bomb Wing (Provisional), active from 1966 until mid 1970.
Hopefully, someone with more knowledge about the unit can provide you more information. Good Luck.