Hey Nate, first off welcome back and thank you for your service.

I don't know much about the Geo/Suzuki 4x4 system but I do have a fair amount of experience with Ford and GM manual locking hubs so they should be similar. Just to rule it out, you're turning the hubs prior to trying to shift into 4H, correct? Hubs turn freely? Does the shifter lever move at all or does it seem bound up? If the body has a fair amount of rust there is certainly the possibility of the linkage binding up. Saturate everything with WD-40 (or your favorite penatrating lubricant) and let it sit. Also, try backing up a short distance prior to trying to engaging. Sometimes the manual systems can get bound up if the PO forced the transfer case to engage without turning the hubs first.

Let me know what you find on your linkage.

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1980 300TD-China Blue/Blue MBTex-2nd Owner, 107K (Alt Blau) OBK #15
'06 Chevy Tahoe Z71 (for the wife & 4 kids, current mule) '03 Honda Odyssey (son #1's ride, reluctantly) '99 GMC Suburban (255K+ miles, semi-retired mule) 21' SeaRay Seville (summer escape pod)