Troopy help - New gearbox etc. But got ping sound on takeoff in first

Petrol or diesel?

Diesel.....ihz ...suggestion is rear uni joint

If you're not totally sure you could drop the rear drive shaft lock the hubs and take it for a spin as a front wheel drive. The last couple I've had replaced developed a click from the extra movement different vehicles to yours.

angyri
Any slight slack on a uni will make the tailshaft ping echo sound as the drive suddenly takes up. If you select reverse and move backward sightly the slack will be ready to sound, Then move forward and the ping will happen again. It is sometimes normal when the vehicle rolls to a stop and the diff is driving the tailshaft, ie any slack is opposite position, When taking off the slack takes up first, My HJ61 used to do it with perfect unis and can be a result of pinion/crownwheel tooth clearance. only takes a small amount and sudden drive to make them ping. Check the unis for any slack in rotation. If you have the transfer in neutral and rotate the pinion flange you will be alarmed at how much slack there rotating slack there is in pinion/crownwheel, diff carrier side gears and axle shaft splines. It all adds up. Easy check done with two large muti grip pliers to feel rotational slack between flange and tailshaft. ie between the yokes to "feel the uni" between.
I presume the slack isn't coming from the gearbox!
How much grease is in the slide joint?????Should be enough or there is no shock absorbing on the dry splines. If lots of K's then the splines will have rotational slackness too. Grease it a bit, see if ping goes away!