The '89 Comanche with the 4.0 (aka the bulletproof straight six 196 CID or so Rambler engine with a few updates-correct me if I'm wrong please) is a 300,000 mile machine from stem to stern if given even the most basic care and should be made into a model studied by budding engineers for successful automotive platforms.
But it isn't a heavy hauler, it is a very, very good utility vehicle and recognizing that limitation is a good on you.
The First Gen Cummins Dodges are awesome trucks and I am in a position to offer you personal opinion and some experience about both transmissions. I have towed exactly the same load with First Gen trucks using both the Getrag and the MV-4500 so perhaps I can offer a more complete view...
My Dad (God rest his soul) bought a '93 D-250 with the Cummins engine and it is in my yard awaiting rebuild. It didn't break, I was an idiot and let my son borrow it and it is now vaguely U shaped from departing pavement for parts unknown.
It will live again-I have a frame for it!
That truck had 200k miles on a Getrag gearbox without rebuild or being repaired and a fair amount of towing a nearly 9000 pound travel trailer that was probably near or at gross most of the time. Dad followed the instructions that materialized as the weakness of that transmission were exposed and were documented in the original print version of the Turbo Diesel Register! In short, if you are at full throttle under load, drop a gear and reduce the power setting. Change the gear oil on schedule and when doing a refill jack the right side of the truck up at least two inches. This allows you to slightly overfill the Getrag box. The cause of the bearing failures was overloading and overheating. By reducing the load and increasing the lubricant volume both issues are avoided.
The Getrag is a mechanical device and has limitations. Operated within those limits it is a very good machine...but don't push it or you will pay!
The MV-4500 is a good transmission and perhaps has better gear ratios (the jump from 2nd to 3rd is not as large as in the Getrag which was always a sticking point with me) but it is not perfect by a long shot, nor is the conversion trivial-I've done it-a W-250 automatic to MV-4500 conversion. At best you will only have to change the bell housing adapter, starter and have the drive shaft length changed-if it is a 4WD like mine, make that both drive shafts!
First off the MV-4500 specs an obsolete gear lube. I only have about 10k miles on an overhauled transmission with the specified gear oil so I cannot say what happens if you put the wrong stuff into it. Some sources say doom on you, some say that something called Royal Purple works fine. I cannot testify to either stance but I do not want my synchromesh to corrode or evolve into goo or whatever other evil is supposed to happen with the wrong lube.
The problem is the specified lube. It is an obsolete Mil-Spec lube that you can't buy at the local AutoYucks store, you have to order it. For about $25 per quart, about 4 or 5 quarts for a fill!
Okay, so you order an extra quart for top offs...still, a hundred bucks for gear oil? I say bovine excrement on that! What ever happened to the days of the T-19 box? Heck, if you had to pee into one of those things to fill it up as long as it didn't run completely dry you couldn't tear one up!
And the 5th gear thing...just WTF is up with that? I dunno, just don't engine brake a Cummins powered Dodge in 5th gear if you have the MV-4500 is the tale that is told. The unmanly and flaccid V-8 diesel Chevy trucks seem to have never rattled the 5th gear loose from it's shaft but there is something about the authority with which the inline 6 Cummins operates when engine braking that unsettles the 5th gear in the MV-4500.
Now can you imagine someone who has only driven a few automatic transmission vehicles (and with distaste at that!) and indeed only owned one for a few months and outside of that wrestling with the not engine braking in 5th gear thing? I pray God that it only applies when towing a heavy load! I don't know how many times I've uttered a string of curses either in my mind or under my breath and slipped that stupid MV-4500 into neutral from 5th so I'm not backloading it going down hill-and that is with one that was rebuilt with all of the supposed updates!
My personal recommendation would be to find a quality shop that has experience with the Getrag and have it rebuilt. Jack the right side of your truck up two or three inches and overfill the thing and drive it like your Great Grandpa would have driving an old steam engine up a grade in the Rockies-like he's afraid the boiler is about to blow and back it off a few degrees...
The First Gen Cummins Dodge trucks would outrun any of it's contemporary diesel or gas trucks up hill back in the day. There is no way that one of these valiant old soldiers can compete with the horrific computerized nightmare of a GM product going up a grade pulling a load 30 years of technological monkeying later-not when those electronic abominations to mechanical glory are putting out three or four times the horse power of the old Dodge!
After an EMP or when their PCM loses it's ground wire or suffers a voltage spike?
Then you can pass their bustedazz fracking computerized wannabe truck with the baby butt smooth ride, low noise interior and smile, secure in the knowledge that in the same way a 1911 is the ultimate sidearm that is not going to let you down, the First Gen Cummins Dodge is the ultimate truck that can make the round trip to Hell and back.
Rebuild the Getrag, going uphill recognize the limitations and man up-drop a gear and slow down...you still have the proper truck for the job if what you care about is getting there regardless of any outside influence. Within their orbit, the First Gens are the Irresistible Force-
Humvee and Land Rover rescue vehicles if you will...
and you will never have to buy any of that tomcat piss DEF stuff!
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