Gearbox

Do you have Hydreco oil Pressure pumps? Check the pressure of the GB oil when you engage it.
There is a pressure adjustment nut on the side of the Hydreco. I have 12V71n's and Allison M20's. The pressure at idle should be 150psi.
I had similar problem on my Allison M20 (port) but only in forward and it turned out to be the pressure pump PLUS this then caused the GB piston & clutch to fail, I had to get a new piston shipped from the US.
From what I understand (which is limited but growing), if the forward gear is the only problem, then the GB can likely be repaired without removing the GB but just pulling it back to access the flywheel, piston and main seal. However if it is the reverse then the GB needs to be removed and rebuilt.
In your case, as the gearbox was overhauled last year, let's hope it is only the pressure pump. It can be rebuilt but a better solution is to buy a new one $1080 and easy to install. They are available in the US, I bought 2 rather than rebuild old ones.
Good luck.

If you are talking about engine oil pressure and rpm's dropping, check to make sure your shaft spins relatively freely and is not binding due to a swollen cutlass bearing or bad alignment. Next would be the engine itself, either it has an internal issue and is not making power, or it needs a tuneup (valve adjustment, injector timing). But I've seen lots of detroits drop a lot of rpm's when you put them in gear........

It does sound like Kafue has the most likely explanation, the pressure pump. Dropping rpm's usually does cause a pressure drop in the gear which will further delay engagement. Since it sounds like the problem was evident before the gear rebuild, the engine could be at fault as well. Have a good Detroit mech check the buffer spring adjustment, it can also show the same symptoms.