popping noise when taking off and sometimes while driving

well come to find out sleepyspeed was right, the u joint in the center was bad, although it wasnt loose i found that the cap had cracked on it. now that brings me to the next problem. i was going to change all the u joints while i had the drive shaft out. i changed the center u joint by removing the retaining clips. the u joint at the trans and the diff, dont appear to have those clips, so i reffered to my haynes manual, and theres a section where it says 86 and later LONG BED models only the center has the retaining clips and if the end ones go bad that portion of the shaft must be changed? however mine is a 92 but its NOT a long bed? the ones on the end appear to have a ring on the top sides of the caps but no gap in the ring to remove it? any ideas or do i just need to put it back in the truck and not worry till one goes bad? surely there is some way to change these things.

Yeah, that is consistent with what I have always been told... can't change either of the front or back u-joints, only the center. I'd never heard it just being the longbeds, although I think the longbeds and cabplus have the same wheelbase.
I've owned a lot of Mazdas over the years and have had a few different mechanics do u-joint replacements told me the same thing. FWIW, I've never needed to replace anything except the center u-joint.
The cab-plus mazdas I have owned seem to eat u-joints faster than standard cabs too when lowered with tall blocks in the rear, but that's just been my experience.

ditto... heard the same here and even had an experience. U-joint went out and I had to replace the drive-shaft (that one was an '86 standard cab short bed on 4" blocks)... found one at a local salvage yard. Then again, that was in 1996 or so. They are getting harder and harder to find parts for... used. I am hoping I don't go thru the same on the 3rd one I have now, a 93 cab plus on 3" blocks. Might end up getting a machine shop to make the u-joints replaceable...