I'm looking to buy a jeep and found a 1990 with straight 6. The guy told me that the vanccum harness/lines for the 4 wheel drive up front has been eaten by battery acid over the years so it won't engage the front axel anymore. i was wondering the seriousness of the problem and the possible price for fixing that, and if it is something an average joe could be able to fix. Thank You.

If it is only the lines - you can fix it on a weekend - with new hoses and
a vacume gauge. You just have to have the right hose and some small clamps.

I'd be more worried about the fact that you can't test it to see if the
t-case and the axle are working OK - the fix is sorta secondary.

My D-30 on my 90 has to be rebuilt because the axle seal on one side leaked all
the fluid out - you would be looking for leaks on either side of the axle in the
bottom of the inner c - with dirt caked on it if it has been there a while.On
the cover or at the tube joints.

Oil leaks on a 90 are prolific - check the valve cover - the main seal - the
side of the head that you can see - and also check for leaks from the bottom
of the power stearing pump and box.

Also - find the place where they park it - and look on the ground.

Instead of replacing the vacuum lines, is it possible to install locking hubs?

Yes - it depends on the specifics of your axle but it is common to
get rid of the vacume system and put a Warn upgrade kit it.

Have not done it myself - so you would have to check out the vendors
and see what is available for your model.

The biggest issue with ANY d-30 upgrade is - is it better just to
get a better bigger axle - and use the upgrade money on that instead
of beefing up a D-30.

if all you are wondering is the difficulty of replacing the vacuum hoses. then it is easy, just buy new hoses from the dealer or any one else. the run between 35 and 80 dollars depending on the year of the jeep.the replacement is pretty straight forward, just unplug the harness from the t-case and the axle and replace making sure the new harness is away from any moving parts and heat. a cable up grade is around 250.

Thanks for all the info guys this helps alot.

I could not buy the kit from DC - and I did not want to
take the time to order it off the net - took the hoses
apart and found that I could replace most of it with hoses
from NAPA - got some good 7 mm hose and - fabbed it myself.