Could be brakes, diffs or gearbox. If you put the 2/4wd lever in its "middle" position it'll totally disengage the front wheel drive, dropping off the propshaft will do the same. Take off the rear propshaft. If it will roll then the gearbox is shagged. If the rear diff is seized you can move the car by removing the half shafts - unbolt the drive flange and pull them out. If it still won't budge you'll have to take the drums off and remove the shoes. Or you might get away with slackening off the adjusters.
very unlikely to be diffs giving problems unless they've been run dry of oil and burnt the bearings out which takes a bit of doing (changed one the other week where a hole had appeared in the diff pan which let the pinion bearing run dry and weld itself up)
middle position on the "high/low" lever disconnects drive from the transfer box thus allowing you to try the 4 gears and reverse operates, the main gearbox can be operated but leaves both axles connected together to give 4 points of resistance (tyre contact) to the ground when the handbrake is applied (this is for PTO operation usually)
being as you have jacked the rear off the floor (i guess both wheels together) and you're struggling to rotate the wheels i'd say that one or both brakes are dragging and possibly the same with the front wheels, both rear wheels off the floor together, rotate one clockwise and the other should then rotate the opposite direction with the handbrake on, take the handbrake off and put the high/low lever forwards so that it's in 4x2 with the main box in neutral and usually the path of least resistance is the handbrake drum rotates when you turn a wheel
you don't say how long you've spent building or whether you have removed the drums and done any checks or servicing in them, it could be rusty drums, poor adjustment, knackered flexis or siezed slaves, i've had drums turn into absolute pigs to get off after having stood for a few years and then rust has grown on the braking surfaces
now i'm not saying i'm correct but i certainly wouldn't start shouting that your gearbox is buggered, think through your problem and understand your driveline before consigning any of it to the bin
the only time i personally have had a gearbox lock up on me is when the castle nut on the rear of the mainshaft came loose, this meant i couldn't get the gearbox out of gear or get the vehicle to move at all, i pulled one halfshaft and it released the tension in the gearbox allowing me to get it out of gear, i then retightened the castle nut and slid the halfshaft back in and all was good (i don't think that little bit of info' has anything to do with your problem)
