Prolly the issue.

But cruising at 3,000 rpm at 75 seems kinda high, like way high. At 70 mph in 4th gear with 2.93's (GTP gears) you should only be at 1900-2000 rpm, at 75 mph maybe 2200-2300. At 3,000 you'd still be 3rd gear going that fast, and that means your 4th gear is gone. I'd do a full rebuild at this point, everything inside the tranny is worn out.