I think there is more to this, i had same problem, new 100% rebuilt by TA , 904, always felt like locked a little when first backing up, OK later; Ok if warmed up too. Not a fluid level thing.. It was as if reverse band was not releasing; Band is always on by spring, remember, it releases to back up ,and clutches (or sprag?) carry torque. So thinking was, it was left partially on when backing up, but only when cold. I thought it would be OK,wear in, but at 17k it was getting worse. I opened trans .rear band was wrecked pieces in pan, some metal chips in pan too, no adjustment; sending back to TA momentarily...I hope they stand behind it. I put in low first and had to go new converter as it had small input spline (early 67 ) ; big bucks to punt at 17k.
There is some syndrome to this; one good trans mechanic said seal rings in front can leak,or similar, not installed right, not enough pressure to release band,
It also would be hesitant about 2-3 shift when cold, throttle linkage perfect. on a slant six ;
related , new converter has scrape marks where it hit on front pump cover,...weird. opposite of above, too far into bell..
Any comments welcome. No email communication at TA can be pain in ***,but hope they come through..