Sorry to start another post on this topic, but I need some help/advice from you guys with similar trucks.
89 GMC Jimmy, 350, TH700, 10 bolts front and rear. currently with what I believe would be stock gears, running on 33's.
I haven't actually removed the diff covers to see whats there but I know I need something different.
It seems a bit sluggish with even the 33's.
I have tried a few different calculators and can't seem to get a decent answer.
I'm going to be doing light offroading, and a fair bit of highway driving especially towing a boat.
It will have 37's and I want good performance and fuel economy without putting stress on anything.

What ratio should i get?
What do most people use for this sort of application?

I say your stock 10bolt axles aren't strong enough to run 37's.

I agree with Darren. I've been thinking about that and depending on what your stock gearing was, your stock tire size was roughly 31s. Given that, I wouldn't go over 35s on those 10 bolt axles, and I'd probably turn them with 4.56 gears. That's plenty of gear for 35s, and with those you probably wouldn't have that sluggish feeling.

You're not doing serious offroading, you'll tow a fair amount, and it'll be mostly highway? Yeah 35s would be nice, 4.56s with the OD tranny when you're not towing would be golden. While towing it's best to put it in 3rd (Direct/Drive) and those 4.56s would give you some good leverage on that boat.

You can probably get away with running your 10 bolts with 37's since you mentioned that you're only going to be doing light wheeling. The fact that your Jimmy is an 89 means that you have the 30 spline axles, which will help some.

Your axle ratio will be in the 3 letter/number codes inside the glove compartment lid. They are RPO codes. Once you write down the codes, you can check them out here.

RPO CODES

F's & G's usually have the gear ratios.

For 37's I would say you would probably be better off with somewhere around 4:56 gears. You will need the "thick" gears if you plan to use your current carriers. Find your ratio, and then use the RPM chart. The charts are ok, but not the best tool. I would at least go one more gear set higher to account for drag/friction from the larger tires. actually, I would bet that the RPM chart tells you that you need 4.10's so... either 4.33 or just go 4.56. Since your RPM are sucking already, using the chart is going to be a little tricky.

Anyway, good luck, and remember to get the thick gears if you go lower than 4.10, or they won't work. Talk to whomever you buy them from, and let them know the situation, they'll know what you need.