Odd issue shifting from reverse to first and rear suspension trashed

[the_guy]

I have a 77 SWB with 3 on the floor and sometimes when I leave it parked for the better part of a day and I go to back out of a parking spot the truck will back up just fine but when I put it in first to drive away it seems to have a lot of strain to the point where it has killed the engine before. It doesn't always do this and I'm not using the parking brake at all when I park it. Once I get rolling and come up to a stop sign I don't have a problems getting moving again. I thought it might be the brakes doing some sort of binding going from reverse to forward after they have been sitting for a while, but that is just a guess. I know the engine mounts are in really poor shape but I can't imagine that would affect it going from reverse to forward and not just plain moving at all. Also, the previous owner decided to put springs in the back that only have 2 leafs each and my rear end is literally sitting on the bump stops. Obviously this has it sitting really low at the moment. I plan to replace these springs asap. I want get some lowering shackles to bring the height back down and I need to know what kind of shocks I will need. This is my first real project so I'm kinda slowly learning everything. Any advice is greatly appreciated.