I have recently purchased a 1984 F150 4x4 with a 302 and pretty big mud tires.
The front hubs have manual locks on them to engage 4 wheel drive, which I understand how to use.
What I'm a little unsure of is when to certainly NOT have them locked in.

Obviously not highway driving, but is there a speed which I should not reach with the hubs locked but the truck in 2WD?

I set up courses for off road marathons and I don't USUALLY need 4WD, but it's handy to have available so I want to know if I can cruise around 20 miles at a time on dirt roads with the hubs locked for the rare cases I need to switch it to 4WD mode to get through mud or inclines.
Then obviously I'd unlock them to drive home

But what are the general perameters for using manually locking hubs? Thanks.

 

You can drive with the hubs locked in 2wd at any time and any speed

 

Quick follow up question..

I bought it from a guy who hadnt driven it in a pretty long time, it runs awesome now that i changed the oil and rubber, but the spark plugs were so old they broke in half, and the oil was jet black...

SO, is there any hub or dofferential maintenence that i should perform??