You have 35" tyres, they came with 29", you have 4.56 gears and an overdrive 5th gear which could be the 0.73 ratio in a NP4500, guess what??
The table below is for 4th gear at 55mph, which is 1:1 and your 35" tyres work OK on this ratio. If you have an 0.73 5th gear you will be pulling 27% less revs and you will struggle if you are pulling 1757rpm at 55mph.
If you had the 0.86 overdrive ratio in a T5 box you would have been pulling 2070 rpm and that is also really sluggish but more manageable.
Sorry, it is your gearing in overdrive top. I would be tempted to steer clear of it unless going downhill as it will bog and use more fuel than 4th which is nicely geared for 35" tyres.
P.S. Dads are always right.
BagusJeep lives in Bali, the Land of Temples.
With a Jeep every prayer counts.
1981 CJ7 258ci - Bagusjeep
1984 CJ7 258ci - Puthijeep
1981 J20 258ci - Gladys
1995 Cherokee 4.0 - CHEROKEE