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This is the location of the check ball capsule in the case
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I have hesitated to comment on this, I know next to nothing about transfer cases. It seems all the obvious stuff had been checked out as ok. Before tearing into the transfer case or transmission, I’d suggest replacing anything with the slightest about of slop. A little slop here or there can add up to a a large amount of slop.
Id be curious what the line pressure is. You need a gauge that reads to 300psi with 1/8 pipe threads. The pressure port is above the shift linkage.
In in drive, with the engine idling, you should see around 90psi, and then smoothly and immediately increase as throttle is added. In reverse, about 125 at idle, once again smoothly and immediately going up with throttle to around 250ish, depending on calibration. If the pressure is much higher at idle, make sure the TV cable is adjusted correctly.
There is a anti-clutch spring on the low/reverse roller clutch support. It’s designed to keep the support preloaded against the case with the transmission in drive. I can tell you I have seen cases so chewed up the support didn’t come out without destroying the case due to severe wear. Usually those transmissions were in trucks with snowplows, or people playing in the mud. I can’t recall anyone complaining about a clunk shifting into reverse, of course by the time the give the transmission to me for repair, a clunk is the least of their problems.
The 4L60 uses two clutches to reverse. The first is the reverse input clutch. It’s the drum right behind the pump, the 2-4 band wraps around and stops the drum from turning in 2nd and 4th gear. GM has changed designs on the pump, drum, and pistons many times over the years. One of the design changes was the size of the feed holes in the pump and drum. I have no clue why they played around with the hole sizes. If you replace a small feed hole pump with one with a large hole, combined with a large hole in the drum, you will get a hard reverse engagement. Replace a large hole pump with a small hole, combined with a small hole in the drum, you will get a slow reverse (or possibly no reverse). They also played around with bleed holes in the piston (designed to allow air to escape the drum. The early drums had a check ball that seated once fluid was applied. Then the eliminated the check ball and drilled a small hole in the piston. The idea of the hole was once the clutch applied, the wavy clutch plate would flatten out and cover the hole, sealing off the orifice. The problem was, in order for the hole to be covered by the flattened wave plate, enough pressure needs to build fast enough to flatten the plate. I’m guessing this is why they played around with pump/drum hole sizes, trying to find a combination that works.
The other clutch needed is the low/reverse clutch. It has a captured checkball in a capsule that’s driven into the case. The check ball is suppose to seat when you shift to reverse, forcing fluid thru a orifice to slow the clutch apply. Once you shift out of reverse, fluid reversed due to the return springs forces the piston to its released position, the fluid unseats the ball and allows fluid to escape quickly.
If this is a new problem, then pump/drum/piston mismatch isn’t a concern. If the transmission has been recently overhauled, then mismatched parts might a possibility. It’s possible for the check ball in the capsule to fall out during cleaning. Some builders remove it to speed reverse engagement. Excessive clutch pack clearance, eliminating the wave plate from the low/reverse clutch can also be a cause. Some builders remove the wave plates to speed clutch apply, definitely not something you would want to do in a daily driver 4x4.
it still did it in 4wd the times I had to move my truck after it broke. Although I will say the rear end was tight, pinion to ring gear, it somehow made the center pin bolt vanish without a trace and dropped the pin, I just associated that mostly to the fact it was a 10 bolt rear, but the trans slamming into gear didn't help at all. 
