I just picked up my new/old '75 F100. I previously owned an all original 8500 mile F250 highboy that I sold a few years ago.
This ol' girl is still in pretty decent original condition - but I've got some work to do. I've been on some pretty deep dives into just about every system on trucks- but automatic transmissions have always been some kind of voodoo cursed pandoras box! She gives a pretty good "clunk" when I hit reverse, but seems to move pretty well into drive. Shifts up and down decently. I wanted to get it serviced/tuned up, but nobody wants to do it. Ford garage - nope, 2x local shops - nope. So, it's gonna be me, YouTube and hopefully some of of you smart people figuring out how to keep this truck on the road for another 47 years.

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Welcome to FTZ. Ditto on "Nice looking rig!".

Don't blame the auto transmission for the clunk yet. Do manually rotate the driveshaft back and forth, engine off, E-brake set, T-case in neutral. With a bit of force each way, feel for anything not smooth or clicking/snapping. Inspect the u-joints, look at all the crosses to see if all show grease, if greaseable joint(s) OR not greaseable look for a cross that might show some powdered rust. Also the slip joint, that grease, when greasing, is going thru the splines and showing at the smooth part of the driveshaft. When engine completely warmed up, that curb idle is set and not idling to high, compared to spec.

Thinking You can do the the tune up. IS the engine stock or modified? What is the engine?

 

I greased her up pretty well on the initial service. All the fluids were low; engine oil like hot fudge, empty power steering pump, and transmission dipstick was dry. She's got all new fluids. I still get the clunk going into (or passing through) reverse, but it's softer. Still shifts smooth. The engine is a '76 460 from a Lincoln. I haven't verified it.

 

Did You do ALL I suggested?? The slip joint tends to let grease go out a small hole at the bottom of the slip. Placing your pinky over the hole while greasing will force the grease thru the splines (thinking this is a 2 piece rear driveshaft ???). There's numerous place's for slop in the drive train. Wanna do a test to eliminate the rear end and rear driveshaft from the problem? Remove the rear driveshaft. engage the front axle to 4 Hi and the lockout hubs. Try your shifting to see if clunk still there. Take it for a ride, all warmed up. Then do all you have that makes the clunk.