Unless things have been swapped around, the transfer case should be a NP208. Searching NP208, you should be able to find photos to confirm your t-case.
There may be a tag on the rear end, if not then you can check other ways.
As for gears: they could be 3.73:1 or several other gearsets... To get an approximate gear ratio; you can put a mark on the driveshaft and one on the wheel. Then jack up the rear axle, rotate the tire one revolution and count the number of times the driveshaft rotates. If the driveshaft rotates almost 3 1/2 times per one wheel revolution, then it's likely 3.42:1. If you're driveshaft rotates about 3 3/4 turns, then it's likely 3.73:1. Just over 4 turns is probably 4.11:1.
Alternatively, you can find the gear ratio by removing the differential cover and counting the teeth on the ring gear and the teeth on the pinion. Ring gear teeth divided by pinion teeth give you the gear ratio.
There were manual transmission Suburbans in 2WD and 4WD, either weren't real common. You'd need the manual transmission components from a pickup, Blazer or Suburban. There are plenty of threads about swapping in a manual transmission in pickups and Blazers.
The SM465 with a NP208 is a fairly common setup in the 80's GM trucks and would present the most straightforward route for a manual transmission in your Suburban.
Besides adding the clutch components, like the pedal setup with either hydraulic or mechanical clutch linkage and swapping in the manual transmission/bell housing; there's also floor pan differences between automatic and manual transmission trucks/Blazer/Suburbans. Additionally, the driveshafts would need shortened/lengthened (depending upon final length of transmission/t-case setup).
Also, there are a lot of people swapping in the later NV4500 5-speed into trucks which, if you chose one of those, would present a few issues of its' own.
Like any auto to manual transmission swap, there are plenty of things to cover.
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