Well the transfer case in the Transcat ( we have one here in the Falklands) is a Spicer T18 as used in Jeeps like the CJ-5 through to the CJ-7, so a search of Jeep related accessories may turn up something..?
I get Petersens $-Wheel & Off-Road magazine & I’ve not seen anything in that which offers overdrives, but that does’nt mean they are not out there..
A company like Advance Adapters may be able to suggest a scource or different gear-sets for the T-18 ( I think the cogs for a T-20 fit in a T-18 case but not absolutely certain about that..)
The axles are Dana items ( 20 on front & 40 on the rear I think it is.? ) also from the CJ range, & there are masses of aftermarket crownwheel/pinion sets available for these..
Alternatively, 8.25×16 tyres such as the XZL Michelin used on Pinzgauers, do fit in the wheel arches & will give a 90-95mm profile casing, & will step you up a little bit..
Of course, you loose a bit of torque with gearset/tyre changes, & speedometer will need checcking & allowance made for differences..!