3500 would make me uncomfortable. Do you drive it like that a lot? If not it's fine but I'd be looking for a OD. Maybe a T5 from a newer Jeep?
The easiest solution is a Warn type OD that bolts to the PTO port on the existing transfer case.About 800 -1000 bucks.Any other transmission is a major drive line reconstruction
3500 rpm isn't that severe in my opinion considering it's just for 15 minutes or so. And in boat use the 153 engines are run at 3500 rpm and high load with no problems.It's not like this engine needs to last 100,000 miles at my age
