B2600i 4x4 with multiple issues

I just was given a B2600i that supposedly needed new fuel injectors. That part has been cleared up, and the injectors were fine. I do have a host of other issues that I haven't been able to find resolutions for, and I hope that someone here might have an answer. They are as follows:
1) Speedometer / Odometer don't work. I know, this isn't wise driving with this going on, but at this time, I go with the flow of traffic, and use the tachometer, which still works to gauge fuel consumption.
2) Fuel economy sucks. I estimated I got about 5 - 7 miles per gallon, after putting 9.5 gallons in the tank at empty, and I was able to drive approximately 90 miles on that fuel. That was mostly highway travel, but at the same time, I get about 5 or less in city driving. I can smell gas when I walk around the vehicle after it has been running, so I'm assured that there is a gas leak. Could this also be an issue that a tune-up would resolve.
3) The computer was replaced at the same time instead of the fuel injectors, and would that have anything to do with the mileage.
4) The HOLD light keeps flashing on the dash. HOLD hasn't been engaged.
5) Serious leaks from the Transfer case on the transmission.

If anyone can help me with troubleshooting, it would greatly appreciated.
Regards,
-Mike

1 speedo check the cable at the trans
maybe loose or broken cable inside
if the cables good then suspect the instrument panel
2 check for leaks that would definatly affect mileage
3 ecm=no
4 mine's a manual trans no help here
5 spray down the transfer case with some degreaser or brake cleaner and check the leak
mine was leaking at the seal for the rear driveshft

Thanks for the info. I just picked up the Haynes manual, with hopes that it would show me the cable to the panel, but alas, no such luck. I'm taking it to a transmission shop next week to check for the leaks and cable.