Are there any good geographical positions the rebels could force loyalists to fight for? Unfortunately I am unfamiliar with Libyan geography. Possibly a hill with good lines of fire that can be prepared with artillery and fortifications at the top? The people could prepare such a place while the fighters keep the attention on the cities. Trenches and tunnels would help. The rebels will have to delay the loyalists until they can occupy a defensive position that has been prepared in the best possible way given the time. After that, it would be best to be prepared to fall back to a similarly prepared position that had been prepared while the rebels delayed the loyalists.
If you have gun crews that can hit a target, and a tank in the open, you have a decent chance to kill it, especially if you can fire first from a hidden position. Also, remember that tanks often have thicker armor in the front. This would be difficult without that damn air support, though. What model tanks do the loyalists have?
However though, it seems that the rebels will need to conserve their strength until reaching Tobruk.
Begin stockpiling water in Tobruk. It has a deepwater port, which could be the rebel's lifeline. It's too bad there wasn't one of those big old battleships attached to the Enterprise.
From wiki: However, Tobruk suffers a serious problem that its stock of underground water is salty. A factory for the desalination of sea water has been built there.