Regular wheel bearing grease is fine, just use the lightweight stuff, and not the "Heavy Duty" grease.
One note tho...if the box has been dry and you've been driving it, it may have already done damage that greasing won't fix.
I would suggest a mixture of 90wt gear lube and wheel bearing grease. Put in an ounce or so of gear lube first, then fill with the grease. This will help lube the balls in the gearbox, if they have been run dry.
If it doesn't fix the steering stiffness, you're only out the cost of a little bit of lube, and time.
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