1989 Transmission? Problem

I have a 6 cyl 4wd Laredo that has me totally perplexed. Occasionally, and there is no fixed time frame, it loses drive. The best description is, if it was a manual and I put my foot on the clutch pedal. This only occurs in top gear, and generally on the freeway. Most times if I take my foot off the gas for a few seconds it 're-engages' and everything is fine, but other times I have to slow down and manually select a lower gear before it gets drive again in top gear. So far I have drained the tranny and replaced the fluid and replaced the filter. Drained and replaced the transfer box fluid, but the problem persists. As I have mentioned, and this is by no means 100% accurate, it occurs mostly when I have driven at high, 70/75 mph with short bursts of up to 80 to overtake, and only when I have been driving for some time and at operating temperature. There is absolutely no warning that anything is amiss before this happens. Any help with my problem would be greatly appreciated.