I assume the Shift Hesitation /Slamming into Gear has gotten worse lately?
Does the Powertraing get overheated quickly/easily now as compared to when new?
Presume you have changed Trans Filters, use the correct viscosity oil for the teamperature range you operate in, checked screens on Powertrain sump, and checked the flex hose going to powertrain pump for collapsing/restriction?
If so, I'd suggest getting someone very familiar with a D4H to do an overall pressure test on the Powertrain Hydraulic System. Its covered pretty well in the Service manual which also has the Specified Pressure levels, but a few memories below:
The slamming into gear indicates a non-modulated shift - -the trans valve spools were probably polished trying to remove burrs.
There is a slight bit of time in a trans shift as one speed is dropped out and another is engaged - -it should not get worse as Temperature gets hotter - - that situation indicates a Leak somewhere in the system, which gets more apparent as oil heats up and gets thinner (reduced viscosity).
In the D4H, the Steering Clutches and Brakes get Priority on Oil pressure & Flow, then Trans P1 (Speed Clutch) and P2 (Trans Directional) Clutch.
The LH & RH Steer Clutches are Pressure Applied, and LH & RH Brakes are Pressure Released / Spring Applied, thus when you are dozing, all 4 are pressurized all the time.
In a Transmission shift, either to change Speed or Direction:
1) The Steering clutches and Brakes always stay applied and get their Oil First!
2) The Trans P1 Speed Clutch fills first, then
3) The Trans P2 Direction Clutch fills.
IF ANY CLUTCH or BRAKE Piston Seals, or the Parking Brake Valve (Which dumps Brake supply oil to both spring applied brakes to engage them) are leaking, the trans engagement will be delayed. It will get worse as oil temp rises.
Fritz, the best way to Find a Leak is by a pressure test done at Engine Low Idle (NOT HIGH IDLE!!)speed with Warmed-up Powertrain oil.
Install gages on all 5 steer valve ports (Supply, LH Clutch, LH Brake, RH Brake, RH Clucth), Trans P1 & P2, and the Pump Port (I think its also on rear of trans cover).
With Machine parked in an open area away from all people and machines, Powertrain Oil Warmed up, and Engine at Low Idle, Watch all gages as as the following steps are done - -you are essentially "cutting off" functions one-by-one so that the Leak is cut off - -which will show itself by the remaining gages going UP in pressure.
1) move the Parking Brake lever from Parked to the Run position and back - - gages may flicker as Park Lever moved but should not change high level.
2) Watch Trans P1 as you slowly move each Steer Lever back through Clutch, then Brake - -you are shutting off flow to each clutch/brake piston as you do this - -and IF P1 Goes up, you have just shut off the leak and need to fix that component.
3) At Engine Low Idle, with Park Brake Released, and foot applied Servine Brake APPLIED, shift trans through and pause for a few seconds in 1F, 2F, 3F, Neutral, 1R,2R, 3R - -again, if Trans P2 varies more than 10-20 psi in any of these gears, it indicates a leak in a Trans Speed or Direction Clutch.
If a "leaker" is found, fix that component.
If Pump Pressure is low and stays low, it may indicate a worn pump or a partially blocked oil sump hose/tube.
And, at 6000 hours, it simply may be time for a trans/torque convertor overhaul.