The drive train of Series Land Rovers use Whitworth bolts and nuts. You can mostly fake having the right size tools with a combination of Metric, AF wrenches and an adjustable spanner. However you risk damaging the heads with improper fitting tools. You are best off having the correct size wrench or socket for the job you are doing.
You do not need a complete socket or wrench set for your truck as there are some sizes that do not exist on your truck. The most common Whitworth sizes for your truck are:
3/16 - The little nuts holding the transfercase bottom plate and I think the retaining bolts for the big swivel housing seal
1/4 - Common use
5/16 - Common use on the transfercase. You will need a 5/16ths socket for the nuts inside the transfercase holding it to the gearbox
3/8 - The transfercase mounting bracket fixings and a couple other places
You might also consider a 7/16 and a 1/2 spanner. They are not as commonly used as the other sizes and you can normally get an adjustable spanner in where these nuts are located.