RRC 1977 after hard braking impossible to drive foreward

Hello,
I recently purchased a 1977 RRC that was in a barn for over 25 years. It has only covered 96000 kilometers so very low mileage. I managed to make the engine turn smooth and reconditionned the brake callippers and all this seems to function well.
Unfortunately when test driving I noticed than when I brake hard, the car enters easily in 1st gear but when I release the clutch, it doesn't move foreward... It shakes and feels like blocked. Only after putting it in reverse and driving a little backwards, it's possible to drive forewards again...
I think the problem is situated in the central differential but far from sure
Also I noticed that there is quite a lot of play in the front driving axle (approx 50°)
Are there any specialists who can give me an advise on where to start. I really hope I don't have to take out the gearbox, because that's beyond my technical capacities...
sorry for my bad English!
Philip
Bruges- Belgium

I'd start by checking the brakes.
I wonder if one of the pads is faulty or there is a problem with the caliper that means the pad is moving and jamming in the caliper against the disc. Reversing moves it back & you're able to drive again.
Don't do this on a public road but using proper brake pipe clamps clamp the flexible hose(s) to each wheel in turn & repeat the hard braking to see what happens.
Bear in mind that the vehicle may well pull to one side whilst doing this so be prepared for it.

With that much free play on the front axle it would be worth draining the oil and check for bits of metal coming out with the oil.or i might be the UJs in the tractor joints.

id remove a prop in turn and try in diff lock

thank all off you who helped!
It were indeed the new brakepads that caused the problem!
I'm happy it was not the transfer or gear box!