Problems moving in Reverse 77 K5

[Spence]

Hey guys

77 GMC Jimmy
non original th350 w/ new torque converter ( I think its out of an 86 that got scraped)
NP 203 part time

Have moved it around property a bunch over the past few months no problems but first drive into town in a few months about 30 miles each way. Seems to be driving and shifting as good as it usually does, but when I pull up to bar and try to parallel park Reverse starts acting really strange.

Put in reverse and let foot off brake- No problem just rolls back

but

Give it gas in reverse and motor starts getting really bogged like you let the clutch out in a high gear and it wants to stall

If I put transfer case in neutral and then transmission in reverse it spins and revs no problem which makes me think this is a rear end problem

coincidentally rear end is all leaky and wet

any thoughts?